After failing his audition for Tough Enough 2, a competition in which the winner earned a World Wrestling Federation (WWF) contract, he was accepted as a cast member for Tough Enough III in 2002, eventually becoming the co-winner with Matt Cappotelli. Morrison, as John Hennigan, competed in the Acolytes Protection Agency's invitational Bar Room Brawl match at Vengeance in July 2003. Hennigan and Cappotelli made an appearance on an episode of Heat in January 2004, losing to Garrison Cade and Mark Jindrak.
On the March 1, 2004 episode of Raw, Hennigan debuted on the main roster as a heel, under the ring name "Johnny Blaze". His gimmick was that he was the apprentice and assistant of the Raw General Manager, Eric Bischoff. The next week, his name was changed to "Johnny Spade", and three weeks after that it was changed to "Johnny Nitro". Nitro acted as Bischoff's apprentice and assistant until June. Nitro lost a match on the June 7 episode of Raw against Eugene, which carried the stipulation that upon his loss, he also lost his job.
The following year, Nitro would return with Joey Mercury and Melina as a group dubbed MNM (Mercury, Nitro, Melina) In their debut match on the main roster on the April 14, 2005 SmackDown, MNM won the WWE Tag Team Championship from Eddie Guerrero and Rey Mysterio. MNM held the titles for three months, having successful titles defenses against the teams of Guerrero and Mysterio, and Charlie Haas and Hardcore Holly, before dropping them to the Legion of Doom at The Great American Bash. They won the titles again in a four-way match defeating Legion of Doom, The Mexicools, and William Regal and Paul Burchill, but lost them to Rey Mysterio and Batista days before Armageddon in December. Due to help from Mark Henry, MNM defeated Mysterio and Batista in a rematch on December 30 to win the championship for the third time. They began a rivalry with the team of Paul London and Brian Kendrick that lasted over three months, until they dropped the championship to London and Kendrick at Judgment Day in May 2006, with the storyline leading to Nitro and Melina suddenly turning on Mercury after the match, breaking up the group. Later that night, Melina and Nitro were booked off the SmackDown! brand by having the General Manager, Theodore Long, "fire" them.
The next week, Nitro, along with Melina, debuted on the Raw brand in a loss to WWE Champion John Cena. Nitro was immediately placed in the Intercontinental Championship picture, beginning his first reign at Vengeance in June after defeating champion Shelton Benjamin in a triple threat match also involving Carlito. Nitro held the belt for four months, feuding with several challengers before losing it to Jeff Hardy on October 2 episode of Raw. Nitro recaptured the Intercontinental Championship from Hardy on the November 6 episode of Raw, when the General Manager for the night, Eric Bischoff, restarted a match that Nitro lost by disqualification. The next week, however, Hardy won the title back. In addition, Nitro was also in a sporadic feud with John Cena throughout the second half of 2006. The feud was unique in that it also later involved non-wrestler Kevin Federline, who appeared on October 16 episode of Raw. Federline was then, in storyline, trained by Nitro for a match with Cena, which he won after interference from Umaga.
In November, MNM reunited on an episode of Raw to accept an open challenge from The Hardy Boyz (Jeff Hardy and Matt Hardy) at December to Dismember, a match that they lost. MNM and The Hardy Boyz feuded across the brands throughout December, with the feud intensifying after Mercury was legitimately injured at Armageddon in a four-way ladder match. Mercury wrestled Matt on SmackDown!, while Nitro continued to feud with Jeff for the Intercontinental Championship on Raw, including a steel cage match at New Year's Revolution. MNM lost to The Hardy Boyz at the Royal Rumble, and again at No Way Out in February which ended the feud. MNM continued on and off as a team until Mercury was released from WWE in March 2007.
Without Mercury, Nitro's association with Melina was played down as she began competing for the WWE Women's Championship. Nitro was paired up with Kenny Dykstra, until the 2007 WWE draft, during which Nitro was sent to the ECW brand and Dykstra was sent to SmackDown. Nitro made his ECW debut on June 19 with a win over Nunzio, and a few days later, on June 24, at Vengeance: Night of Champions, he won the vacant ECW World Championship in a match against CM Punk, making it his first world championship in WWE, when he was called upon to replace Chris Benoit. A few weeks after getting the title, Nitro's ring name was changed to John Morrison. After defeating CM Punk again at The Great American Bash, he began taking part in a "15 Minutes of Fame" segment on the weekly ECW on Sci Fi television program, where if a wrestler could either defeat him or last 15 minutes without being defeated, they would earn a title shot. The first wrestler to do so was CM Punk, which set up another match between the two at SummerSlam. Morrison won by illegally using the ring ropes for leverage. The next week, Punk again became the number one contender; this time, however, Punk won the championship. Morrison was then suspended for thirty days for violating WWE's Wellness Program, renewing his feud with Punk upon his return, as well as competing against The Miz for the number one contendership to the title.
Though rivals, Morrison and The Miz faced the dysfunctional tag team of Matt Hardy and Montel Vontavious Porter on the November 16 episode of SmackDown! for the WWE Tag Team Championship in a winning effort, marking Morrison's fourth WWE Tag Team Championship reign. At Survivor Series, Morrison and The Miz were both defeated by CM Punk in a triple threat match for the ECW Championship. After the defeat, the rivalry aspect of Morrison and Miz's relationship faded, as they were now portrayed as trusting friends. In February 2008, Morrison and Miz were given a streaming segment on WWE.com named The Dirt Sheet in which they mocked other wrestlers and facets of pop culture. The team had many successful title defenses over the next few months, before dropping the titles to Curt Hawkins and Zack Ryder at The Great American Bash in a fatal four-way match which also featured Jesse and Festus, and Finlay and Hornswoggle. Neither Morrison nor Miz were pinned, as Hawkins pinned Jesse to win the titles.
Afterwards, they started a feud with Cryme Tyme (JTG and Shad) through each team's Internet show, which then transitioned to matches on Raw and at Cyber Sunday. After winning the Slammy Award for Best WWE.com exclusive earlier in the day, Morrison and Miz won the "Tag Team of the Year" Slammy Award on December 8, 2008 episode of Raw. On December 13, Morrison and Miz won the World Tag Team Championship from CM Punk and Kofi Kingston at a live event. Morrison and Miz engaged themselves in a feud with The Colóns (Carlito and Primo), who were the reigning WWE Tag Team Champions, which resulted in both teams defending their respective championships successfully in separate matches. Their feud culminated in a tag team lumberjack match where both titles were on the line at the WrestleMania XXV, but it was The Colóns who would unify the championships with a win. On April 13, as part of the 2009 WWE draft, The Miz was drafted to the Raw brand to split up the team, and as a result, Miz attacked Morrison.
On April 15, Morrison returned to the SmackDown brand as part of the 2009 Supplemental Draft. In Morrison's first match back for the brand, on April 17, he defeated R-Truth. On the May 1 episode of SmackDown, Morrison engaged in a backstage fight with Chris Jericho after Jericho slapped him, turning face in the process for the first time in his career. Later that same night, Morrison defeated Shelton Benjamin, beginning a feud between the two. On the May 7 episode of Superstars, Morrison lost a match to Jericho after a distraction by Benjamin. Morrison continued to score victories over Benjamin in singles and tag team matches before defeating him at Judgment Day. During this time, Morrison would soon become wildly popular with fans. On the June 19 episode of SmackDown, Morrison lost a critically acclaimed match against Edge, in which the commentators described it as a "third main event". On the July 31 episode of SmackDown, Morrison had his first World Heavyweight Championship match against Jeff Hardy, but failed to win the title.
On September 4 episode of SmackDown, Morrison defeated Rey Mysterio to win his third Intercontinental Championship. Morrison then began a feud with Dolph Ziggler, after losing a non-title match to him by countout, but he would successfully defend the title against Ziggler at Hell in a Cell, and again on the following episode of SmackDown. The feud between Morrison and Ziggler continued throughout the next month, with the two wrestling to a double countout for the Intercontinental Championship on the November 13 episode of SmackDown, and Morrison retaining the championship in a two-out-of-three falls match the following week to end the feud. At Bragging Rights, Morrison was defeated by his former tag team partner then-United States Champion The Miz in an interbrand Champion vs. Champion match. Their rivalry continued after both were announced as opposing team captains for Survivor Series, where Miz's team defeated Morrison's team. Morrison then went on to feud with newcomer Drew McIntyre, who defeated Morrison in a non-title match on the December 4 episode of SmackDown, thus earning a championship opportunity against Morrison. At the TLC: Tables, Ladders & Chairs pay-per-view, Morrison lost the Intercontinental Championship to McIntyre, ending his reign at 103 days. He received his rematch on the first SmackDown of 2010, but was disqualified when McIntyre shoved him into the referee. He challenged McIntyre for the Intercontinental Championship again on the January 29 episode of SmackDown in a No Disqualification match, but lost, ending their feud. Morrison later won an Elimination Chamber qualifying match and participated in the eponymous match for the World Heavyweight Championship, but was eliminated by The Undertaker. On the March 5 episode of SmackDown, Morrison teamed with R-Truth to defeat Cryme Tyme and The Hart Dynasty to earn a Unified WWE Tag Team Championship match against ShoMiz (Big Show and The Miz). At WrestleMania XXVI, they failed to win the championship.
On April 26, Morrison was drafted to the Raw brand as part of the 2010 WWE draft, and lost his first match back for the brand later that night to Jack Swagger. Later that week, in his final match on SmackDown, he lost to Cody Rhodes. On June 1, Morrison announced that he would be mentoring Eli Cottonwood in the second season of NXT. Cottonwood, however, was the second person eliminated from the competition, and was eliminated on July 27 episode of NXT. Morrison then went on to participate in the main event of SummerSlam in which he and six teammates (John Cena, Edge, Chris Jericho, Bret Hart, R-Truth, and Daniel Bryan) were victorious in a seven-on-seven elimination tag team match against The Nexus. In October 2010, Morrison began a feud with Sheamus after he helped Santino Marella, earn an upset win during a one-sided match against Sheamus. Within the following weeks, Morrison continued to save Marella from attacks by Sheamus. At Survivor Series, Morrison defeated Sheamus. Morrison entered the 2010 edition of the King of the Ring tournament, defeating Tyson Kidd, Cody Rhodes, and Alberto Del Rio. Morrison lost to Sheamus in the final round. He continued to feud with Sheamus and defeated him in a WWE Championship number one contender's ladder match at the Tables, Ladders & Chairs pay-per-view on December 19. Morrison received his WWE Championship match on January 3, 2011 episode of Raw, but lost to reigning champion The Miz in a falls count anywhere match. On January 30, Morrison participated in the Royal Rumble match at the Royal Rumble event and despite not winning, he had a stand-out moment when Morrison was knocked off of the ring apron and performed a cat leap onto the barricade to avoid having his feet touch the floor and then went on to precision jump onto the ring steps and re-enter the ring. On February 20, Morrison participated in the Elimination Chamber match at the Elimination Chamber pay-per-view event, but was eliminated by CM Punk. During the match, Morrison climbed to the top of the cage while inside the cage before dropping to deliver a crossbody on and eliminate Sheamus. At WrestleMania XXVII, Morrison teamed with Trish Stratus and guest star Snooki in a mixed tag match to defeat the team of Dolph Ziggler and LayCool (Layla and Michelle McCool).
On the April 11 episode of Raw, Morrison participated in a five-man gauntlet match for the number one contender's spot for the WWE Championship, but was eliminated by R-Truth. The next week on Raw, Morrison challenged R-Truth to a match later on in the night, with the stipulation that if Morrison won, he would take Truth's place in the steel cage match at Extreme Rules. Truth accepted, and lost the match, making Morrison the new number one contender. After the match, Truth viciously attacked Morrison. At Extreme Rules, R-Truth interfered in the WWE Championship steel cage match, attacking Morrison and preventing him from winning the match. The next night on Raw, Morrison was attacked by Truth again, prior to a scheduled match between the two which did not happen as a result. Morrison then underwent surgery to correct a legitimately pinched nerve in his neck. Morrison initially returned on the June 13 episode of Raw, but Truth attacked him again prior to their scheduled match, re-injuring him. Morrison returned again on July 25 episode of Raw, attacking R-Truth. At SummerSlam, Morrison teamed up with Kofi Kingston and Rey Mysterio to defeat R-Truth, Alberto Del Rio and The Miz. The following night on Raw, Morrison defeated R-Truth in a falls count anywhere match to end their feud. At Night of Champions, Morrison competed in a Fatal 4-Way match for the United States Championship, also involving Alex Riley, Dolph Ziggler, and Jack Swagger, with Ziggler successfully retaining his title. Morrison then challenged Cody Rhodes for the Intercontinental Championship at the Hell in a Cell pay-per-view, but failed to win the title. Following this, Morrison went on a losing streak, lasting a number of weeks. On the November 7 episode of Raw, Morrison picked up his first victory in three months, defeating United States Champion Dolph Ziggler in a non-title match. Morrison then challenged Ziggler for the championship at Survivor Series, but was defeated by Ziggler.
In his last advertised WWE appearance on the November 28 episode of Raw, Morrison faced The Miz in a Falls Count Anywhere match; Miz won the match by delivering the Skull-Crushing Finale on the steel entrance stage, giving Morrison a kayfabe injury. This was used to write Morrison off television, even being stretchered out of the arena after the match. On November 29, WWE acknowledged the end of Morrison's contract, and moved his profile to the alumni section of its website.
On September 26, 2019, it was reported that Morrison had re-signed with WWE, being confirmed on December 3 on WWE Backstage. Morrison made his first WWE television appearance on the January 3, 2020 episode of SmackDown in a brief backstage interview, coming out of The Miz's locker room. On the January 10 episode of SmackDown, Morrison reunited with The Miz on Miz TV and, though he shook hands with the fans as he came to the ring, insulted the audience for booing his former tag team partner, establishing himself as a heel for the first time in WWE since 2009. Subsequently, Morrison helped Miz win his match against Kofi Kingston. Morrison made his in-ring return the following week, defeating Big E. At the Royal Rumble, Morrison participated in the Royal Rumble match as the fifth entrant, but he was eliminated by Brock Lesnar. At Super ShowDown, Miz and Morrison won the SmackDown Tag Team Championship from The New Day.
On March 8, Morrison and Miz had their first title defense against five other teams in the second-ever tag team Elimination Chamber match at the Elimination Chamber event, eliminating The New Day and The Usos at the end of the match to retain the title. At WrestleMania 36, after the Miz's absence due to illness, Morrison defended the SmackDown Tag Team Championships by himself in a ladder match against Kofi Kingston and Jimmy Uso, in which he was successful. On the April 17 episode of SmackDown, the duo lost the titles back to The New Day after The Miz unsuccessfully defended the titles by himself in a triple threat match against Big E and Jey Uso. At Money in the Bank the following month, Morrison and Miz unsuccessfully attempted to regain the championship in a fatal four-way tag team match also involving Lucha House Party (Gran Metalik and Lince Dorado) and The Forgotten Sons (Steve Cutler and Wesley Blake). Next, Morrison and Miz started a rivalry with Universal Champion Braun Strowman. At Backlash, the duo competed for Strowman's title in a two-on-one handicap match, but lost.
As part of the 2020 draft in October, both Morrison and Miz were drafted to the Raw brand. At the Elimination Chamber event on February 21, 2021, Morrison defeated Ricochet, Elias, and Mustafa Ali in a fatal four-way match for an opportunity at the United States Championship. Later that night, he failed to win the title in a triple threat match against Riddle and reigning champion Bobby Lashley. After this, Morrison and Miz feuded with rapper Bad Bunny and Damian Priest. At WrestleMania 37 on April 10, Morrison and Miz faced Bad Bunny and Priest in a tag team match, but lost. After The Miz suffered an injury at WrestleMania Backlash, Morrison would embark on singles run, starting by defeating Jeff Hardy on the June 14 episode of Raw. The following week, Morrison would defeat Randy Orton to qualify for the Money in the Bank ladder match at the namesake pay-per-view event, which was won by Big E. On the August 23, 2021 episode of Raw, The Miz was defeated by Xavier Woods. After the match, The Miz turned on Morrison by attacking him, turning Morrison face in the process. On November 18, 2021, Morrison was released from his WWE contract.