After weeks of vignettes, Gunn made his WWF debut on the May 17, 1993 episode of Raw, teaming his on-screen brother, Bart Gunn to defeat Tony Vadja and Glenn Ruth. The duo, now known as The Smoking Gunns, made their pay-per-view debut at King of the Ring, teaming with The Steiner Brothers to defeat Money Inc. and The Headshrinkers in an eight-man tag team match. At SummerSlam, the duo teamed with Tatanka to pick up a win against Bam Bam Bigelow and the Headshrinkers. On January 22, 1994, Gunn entered his first Royal Rumble match at the namesake event, but was eliminated by Diesel. In early 1995, the Gunns won their first Tag Team Championship by defeating the makeshift team of Bob Holly and 1-2-3 Kid. They held the title until WrestleMania XI, where they were defeated by the team of Owen Hart and Yokozuna. They won the titles again in September 1995.
On February 15, 1996, the Gunns vacated the title because Billy was in need of neck surgery. After Billy returned from hiatus, The Smoking Gunns won the Tag Team Titles for the third time by defeating The Godwinns in May. After the match, The Godwinns' manager Sunny turned on her team in favor of the Gunns. On September 22 at In Your House: Mind Games, the Gunns lost the Tag Team Titles to Owen Hart and The British Bulldog. After the match, Sunny abandoned The Gunns, saying that she would only manage title holders. Billy, frustrated with losing both the championship and Sunny, walked out on Bart, breaking up The Smoking Gunns.
After The Smoking Gunns disbanded, Gunn took some time off to nurse an injury. At WrestleMania 13, he defeated Flash Funk, catching the attention of The Honky Tonk Man, who made Gunn his protégé. During this time, he adopted a new gimmick, Rockabilly, and had a short-lived feud with "The Real Double J" Jesse James. On Shotgun Saturday Night, James realized both of their careers were going nowhere and suggested that they became a tag team. Gunn agreed and smashed a guitar over the Honky Tonk Man's head to solidify their new alliance.
James and Rockabilly were quickly rebranded as "Road Dogg" Jesse James and "Badd Ass" Billy Gunn, respectively, and their tag team was dubbed the New Age Outlaws. They quickly rose to the top of the tag team ranks and won the Tag Team Championship from the Legion of Doom on November 24. They also defeated the LOD in a rematch at In Your House: D-Generation X.
The Outlaws slowly began to align themselves with D-Generation X. At the Royal Rumble, the New Age Outlaws interfered in a casket match to help Shawn Michaels defeat The Undertaker. At No Way Out Of Texas, the Outlaws teamed up with Triple H and Savio Vega (who replaced the injured Shawn Michaels) to face Chainsaw Charlie, Cactus Jack, Owen Hart, and Steve Austin. They were, however, defeated. On February 2, The Outlaws locked Cactus and Chainsaw in a dumpster and pushed it off the stage. This led to a Dumpster match at WrestleMania XIV where Cactus and Chainsaw defeated the Outlaws for the Tag Titles. The next night on Raw, the New Age Outlaws won the Tag Team Championship for a second time by defeating Chainsaw and Cactus in a Steel cage match, but only after interference from Triple H, Chyna, and X-Pac. After the match, the Outlaws officially became members of D-Generation X (DX).
After joining DX, the Outlaws successfully defended their Tag Team Title against the Legion of Doom 2000 at Unforgiven. DX began to feud with Owen Hart and his new stablemates, The Nation. At Over The Edge, the Outlaws and Triple H were defeated by Nation members Owen, Kama Mustafa, and D'Lo Brown in a six-man tag match.
During this time, the Outlaws began a feud with Kane and Mankind. At SummerSlam, Mankind faced the Outlaws in a handicap match after Kane no-showed the title defense. The Outlaws defeated Mankind to win the titles for the third time. In December, the Outlaws lost the title to The Big Boss Man and Ken Shamrock of The Corporation.
The Outlaws then began to focus more on singles competition. The Road Dogg won the Hardcore Championship in December 1998, and Gunn set his sights on the Intercontinental Championship. At the 1999 Royal Rumble, Gunn unsuccessfully challenged Ken Shamrock for the Intercontinental Title. The next month at St. Valentine's Day Massacre, Gunn was the special guest referee for the Intercontinental Championship match between Val Venis and champion Ken Shamrock, where Gunn made a fast count and declared Venis the new champion before attacking both men.
In March, Gunn won the Hardcore Championship from Hardcore Holly. At WrestleMania XV, Gunn lost the title to Holly in a Triple Threat match which also included Al Snow. The New Age Outlaws then reunited to defeat Jeff Jarrett and Owen Hart at Backlash. After Backlash, Gunn left D-Generation X and aligned himself with Triple H and Chyna. Gunn defeated his former partner, Road Dogg, in a match at Over the Edge. Gunn then won the 1999 King of the Ring tournament by defeating Ken Shamrock, Kane, and his former ally, X-Pac. After King of the Ring, Gunn, Triple H, and Chyna went on to feud with X-Pac and Road Dogg over the rights to the D-Generation X name. This feud culminated at Fully Loaded when X-Pac and Road Dogg defeated Gunn and Chyna.
Gunn then began a brief feud with The Rock. At SummerSlam, The Rock defeated Gunn in a Kiss My Ass Match. Following this, Gunn then briefly feuded with Jeff Jarrett for the Intercontinental Title before reuniting with Road Dogg to reform The New Age Outlaws. The Outlaws won their fourth tag team championship by defeating The Rock 'n' Sock Connection in September 1999. The Outlaws later reunited with X-Pac and Triple H to reform D-Generation X. During this time, The Outlaws won their fifth Tag Team Championship after defeating Mankind and Al Snow. At the 2000 Royal Rumble, The New Age Outlaws retained their title against The Acolytes after interference from X-Pac. The Outlaws then had a feud with The Dudley Boyz, who won the Tag Team Championship from The Outlaws at No Way Out. After suffering a torn rotator cuff in the match with The Dudley Boyz, Gunn was kicked out of D-Generation X for "losing his cool" to explain his impending absence to recover from his injury.
Gunn made his return in October and immediately teamed with Chyna to feud with Right to Censor, who wanted to "censor" his Mr. Ass gimmick. At No Mercy, Right to Censor members Steven Richards and Val Venis defeated Chyna and Gunn. Due to a stipulation, Gunn could no longer use the Mr. Ass gimmick, so he renamed himself Billy G. for a few weeks before settling on "The One" Billy Gunn. Gunn then feuded with Eddie Guerrero and the rest of The Radicalz. At Survivor Series, Gunn teamed with Road Dogg, Chyna, and K-Kwik in a losing effort against The Radicalz. A few weeks later on SmackDown!, Gunn won the Intercontinental Championship from Guerrero. However, the title reign was short lived, as Chris Benoit defeated him for the title two weeks later at Armageddon. After feuding with Benoit, Gunn participated in the 2001 Royal Rumble where he made it to the final four. Gunn interfered in the Hardcore Championship Match at No Way Out, and, taking advantage of the 24/7 Rule, pinned Raven for the title. The reign was short-lived, as Raven won it back a few minutes later.
In a 2001 match on Sunday Night Heat, Gunn was defeated by Chuck Palumbo, who recently left The Alliance to join the WWF. After the match, Gunn suggested that they form a tag team. Palumbo agreed, and Billy and Chuck quickly rose to the top of the tag team division. Initially they were a generic tandem, but they were given a gimmick where they grew increasingly affectionate toward each other, showing evidence of a storyline relationship.
In February 2002, Billy and Chuck defeated Spike Dudley and Tazz to win the Tag Team Championship for the first time as a team. After winning the titles, Billy and Chuck found a "Personal Stylist" in the ambiguously flamboyant Rico. After retaining the title against the Acolytes Protection Agency, the Dudley Boyz, and the Hardy Boyz in a Four Corners Elimination Match at WrestleMania X8 and against Al Snow and Maven at Backlash, Billy and Chuck began a feud with Rikishi. At Judgment Day, Rikishi and Rico (Rikishi's mystery partner of Mr. McMahon's choosing) defeated Billy and Chuck for the Tag Team Titles after Rico accidentally hit Chuck with a roundhouse kick. Billy and Chuck quickly won the title back two weeks later on SmackDown! with Rico's help. They held the championship for almost a month before losing it to the team of Edge and Hulk Hogan.
On the September 5 edition of SmackDown!, after Gunn lost a match to Rey Mysterio, Chuck proposed to Billy, asking him to be his "partner for life" and gave him a wedding ring. Gunn agreed, and one week later, on the September 12 episode of SmackDown!, Billy and Chuck had their wedding ceremony. However, just before they tied the knot, they revealed that the entire ordeal was a publicity stunt and disavowed their on-screen relationship, admitting that they were just friends. The "preacher" revealed himself to be Raw General Manager Eric Bischoff (who was wearing a skin mask), who then summoned 3-Minute Warning to beat up Billy and Chuck. Rico, furious that Billy and Chuck gave up their gimmick, became the manager of Three Minute Warning and defected to Raw. At Unforgiven, Three Minute Warning defeated Billy and Chuck. Their final match together occurred on SmackDown! in the first round of a tournament for the newly created WWE Tag Team Championship. They lost the match to the team of Ron Simmons and Reverend D-Von. Afterwards, Gunn took a few months off because of a shoulder injury and the team of Billy and Chuck quietly disbanded.
After returning in the summer of 2003, Gunn reverted to the "Mr. Ass" gimmick, defeating A-Train, and Torrie Wilson became his new manager. He started a feud with Jamie Noble, which led to an "Indecent Proposal" Match at Vengeance, which Noble won and due to the match's stipulation, won a night with Torrie. After taking time off again due to a shoulder injury, Gunn returned to action at the 2004 Royal Rumble, but was eliminated by Goldberg. Afterward, he wrestled mainly on Velocity, forming an occasional tag team with Hardcore Holly. At Judgment Day, Gunn and Holly challenged Charlie Haas and Rico for the Tag Team Championship, but were unsuccessful. At The Great American Bash, Gunn lost to Kenzo Suzuki.
On November 1, 2004, Gunn was released from his WWE contract.
On July 23, Gunn made his first WWE appearance in nearly eight years as he reunited with Road Dogg, X-Pac, Shawn Michaels and Triple H to reform D-Generation X for one night only on the 1000th episode of Raw. In December 2012, he was hired by WWE as a trainer for the NXT brand in Tampa, Florida. On March 4, 2013, Gunn and Road Dogg made a return at Old School Raw, defeating Primo and Epico. On March 11, 2013, they accepted a challenge from Team Rhodes Scholars and faced them in a match, which was interrupted by Brock Lesnar, who hit both Outlaws with an F-5 as part of his ongoing feud with Triple H.
He then appeared alongside Road Dogg to help CM Punk clear out The Shield in aid of Roddy Piper on Old School Raw on January 6, 2014. On the January 10 episode of SmackDown, the Outlaws teamed with CM Punk in a six-man tag match against The Shield in a losing effort. On the January 13 episode of Raw, the Outlaws again teamed with Punk in a rematch against The Shield, only to abandon Punk and lose the match. On January 26 during the Royal Rumble kickoff Show, Gunn and Road Dogg beat Cody Rhodes and Goldust to win the WWE Tag Team Championship. The next night on Raw, the New Age Outlaws retained the championship against Rhodes and Goldust via disqualification when Brock Lesnar attacked the brothers. The next week on Raw, the New Age Outlaws retained the championship against Rhodes and Goldust in a steel cage match. On March 3, the Outlaws lost the Tag Team Championship to The Usos. Gunn sustained hemoptysis after he and his New Age Outlaws partner, Road Dogg, suffered a double-Triple Powerbomb by The Shield at WrestleMania XXX.
Gunn returned to Raw with Road Dogg in January 2015, attacking The Ascension along with the nWo and the APA. At the Royal Rumble, the Outlaws faced The Ascension in a losing effort. At WrestleMania 31, Gunn, with Road Dogg, X-Pac and Shawn Michaels, reunited as D-Generation X to help Triple H in his match against Sting. In May, Gunn was announced as a coach along with WWE Hall of Famers Booker T and Lita for the sixth season of Tough Enough.
On November 13, 2015, WWE officially announced that Gunn was released from his WWE contract.
Gunn and numerous other WWE legends appeared on the January 22, 2018 episode of Raw 25 Years as part of the D-Generation X reunion. He was inducted into the WWE Hall of Fame Class of 2019 as a member of D-Generation X.