Duggan was signed to the WWF in January 1987, making his first major appearance with the WWF at WrestleMania III, where he also made his pay-per-view debut. In the weeks preceding WrestleMania, Duggan began his first WWF feud with Nikolai Volkoff. At the event, he ran to ringside prior to a match between The Killer Bees and The Iron Sheik and Nikolai Volkoff and interrupted Volkoff's pre-match singing of the Russian national anthem. Duggan had previously done this on TV matches leading up to the event.
Although he did not win any major titles in the promotion, for seven years he was a consistent fan favorite with his patriotic gimmick. His character was mainly comical, as he rarely was in major contention for titles. His first major WWF match was at the first-ever Survivor Series pay-per-view in 1987, when he participated in the first Survivor Series match and was a member of the winning team.
By the late fall of 1987, Duggan received his first major push since his return, being placed in a feud opposite "King" Harley Race, who was using a monarch gimmick who sneered at wrestlers he thought were below him. Duggan vowed he would never give in to Race's demands to bow to him, and during a televised confrontation, took Race's crown and cape, though Race would later get them back. At the 1987 Slammy Awards, where Duggan presented the nominees for "Best Ring Apparel," an on-stage shoving match between him and the winner (Race), a wild brawl broke out between the two, going backstage and eventually reaching back to the main stage before interrupting another award presentation and Gorilla Monsoon stepping in to separate the two. Duggan eventually defeated Race several times in both televised and non-televised shows.
Duggan participated in the first-ever Royal Rumble match in 1988, where he was the 13th of twenty entrants. He won the match after he last eliminated the One Man Gang.
At WrestleMania IV, he participated in a 14-man tournament for the vacant WWF World Heavyweight Championship, where he lost to Ted DiBiase in the first round after interference from André the Giant. Shortly after WrestleMania IV, Duggan—upset that André cost him his match—came to the ring during one of André's matches and challenged him; a feud erupted when André suddenly grabbed Duggan and began throttling him, before Duggan knocked André out with his 2x4 board. André generally came out the winner in their matches, usually by interference from André's manager Bobby Heenan. André also interfered in several of Duggan's matches against other wrestlers, including Hercules on the April 30 Saturday Night's Main Event XVI.
Duggan earned a shot at the Intercontinental Heavyweight Championship against The Honky Tonk Man in July 1988, but Honky Tonk got himself disqualified, thus retaining the title.
Duggan then began displaying traits of an American patriot by feuding with several foreign villains in the fall of 1988. He got involved in a feud with Dino Bravo, and the two were on opposing sides as Jake "The Snake" Roberts' team fought André the Giant's at the 1988 Survivor Series. Duggan was disqualified after he used his 2x4 on Bravo. Duggan's team eventually lost the match. His next feud was with Russian Boris Zhukov, whom he defeated in a flag match on the November 26 Saturday Night's Main Event XVIII. He restarted his feud with Dino Bravo, and at the 1989 Royal Rumble, Duggan and The Hart Foundation (Bret Hart and Jim Neidhart) defeated Bravo and The Fabulous Rougeau Brothers (Jacques and Raymond) in a two out of three falls match. He was involved in a rivalry with Bad News Brown, which culminated in a match at WrestleMania V where both men fought to a no contest.
In mid-1989, he started a feud with King Haku, who was awarded the crown on the edition of July 9, 1988 of Superstars when Harley Race was forced to vacate the crown due to injuries sustained in a match against Hulk Hogan on the edition of March 12, 1988 of Saturday Night's Main Event XV. Duggan defeated Haku to win the crown and earn the title of "King of the WWF" on the edition of May 13, 1989 of Superstars. On the May 27 Saturday Night's Main Event XXI, Duggan earned a shot at the Intercontinental Championship against "Ravishing" Rick Rude. Duggan won the match by countout and Rude retained the title, as a title cannot change hands via countout or disqualification. At SummerSlam, Duggan teamed with Demolition (Ax and Smash) in a six-man tag team match and defeated André the Giant and The Twin Towers (Akeem and The Big Boss Man).
He eventually lost his crown to "Macho Man" Randy Savage, who began calling himself "Macho King", and started a feud with Savage. At Survivor Series, he captained a team dubbed "The 4x4's" against Savage's team, "The King's Court". Although Duggan eliminated Greg Valentine with his 3-Point Stance Clothesline, Duggan's team lost the match. He faced Savage again on the January 27 Saturday Night's Main Event XXV. Duggan defeated another former rival, Dino Bravo at WrestleMania VI. He began teaming with his former rival, Nikolai Volkoff and started a feud with The Orient Express (Tanaka and Sato), whom they beat at SummerSlam. He began a short feud with WWF World Heavyweight Champion Sgt. Slaughter and faced Slaughter for the title on the February 1 The Main Event V. Slaughter retained the title by getting himself disqualified.
In late 1991, he patched up things with Sgt. Slaughter, who had turned babyface, and the pair formed a tag team. At Survivor Series, the duo joined the Texas Tornado and Tito Santana in an elimination match to defeat a team led by Col. Mustafa. On the February 8 Saturday Night's Main Event XXX, Duggan and Slaughter defeated the Beverly Brothers (Blake and Beau). At WrestleMania VIII, he teamed up with Virgil, Sgt. Slaughter, and Big Boss Man against The Nasty Boys (Jerry Sags and Brian Knobbs), The Mountie, and Repo Man. Slaughter eventually left active wrestling and Duggan continued to pursue his singles career again.
In mid-1992, Duggan was moved to undercard status, working with Repo Man in house shows. In early 1993, he was involved in a feud with the then near-600 pound Yokozuna, who was the number one contender to the WWF Championship. Manager Mr. Fuji was bad mouthing America and Duggan challenged his protégé to a match. The match was promoted by claiming that no one had knocked Yokozuna off his feet, even though Yokozuna had fallen off his feet shortly before, during the 1993 Royal Rumble match. The stipulations to the match were if Duggan knocked Yokozuna off his feet, Duggan would win. On the edition of February 6 of Superstars, Duggan won the match after knocking Yokozuna down completely, but then was manhandled by the much bigger Yokozuna after the match and fell victim to four Banzai Drops, sidelining Duggan for about four months.
Duggan eventually returned to television and started a feud with then-Intercontinental Champion Shawn Michaels, challenging him for the title on the edition of May 3 of Monday Night Raw. Michaels ran through the crowd, and retained the title by countout. On the next show, to prevent Michaels from pulling the stunt again, Duggan received a return bout in a lumberjack match which Michaels won on a disqualification. As Michaels and the lumberjacks brawled after the match, Duggan came to and cleared the ring with his 2-by-4. Duggan then competed in the King of the Ring tournament, defeating Papa Shango to qualify before losing to Bam Bam Bigelow in the quarterfinals. He continued to appear on WWF programming until the summer, when he left the company after losing several pre-SummerSlam matches to Yokozuna.
Duggan signed a contract with World Championship Wrestling (WCW) in the summer of 1994. He debuted on the edition of August 6 of WCW Saturday Night, defeating Rip Rogers. A month later at Fall Brawl, as a replacement for the injured Ricky Steamboat, he defeated Steve Austin in a squash match lasting thirty-five seconds to win the United States Heavyweight Championship. After becoming champion, he quickly started a feud with Austin, and eventually defeated him in a title rematch at Halloween Havoc by disqualification. The two had a second title rematch on the November 16 Clash of the Champions XXIX, where Duggan retained the title again by disqualification. Duggan's reign ended at Starrcade, where he was defeated by Vader.
After he lost the U.S. title to Vader, Duggan moved to the mid-card division and began wrestling on Saturday Night. He defeated Bunkhouse Buck at SuperBrawl V. He faced former WWF rival Meng (known as Haku in WWF) in a martial arts match at Uncensored, which Meng won. At The Great American Bash, he defeated Sgt. Craig Pittman by disqualification in Pittman's pay-per-view debut. At Bash at the Beach, Duggan lost to Kamala.
In late 1995, Duggan started a feud with Big Bubba Rogers, which culminated in a Taped Fist match at World War 3, which Rogers won. Duggan participated in the first-ever three ring, 60-man battle royal for the vacant World Heavyweight Championship, but was eliminated. He fought Loch Ness to a double disqualification at a Main Event taping before SuperBrawl VI. Duggan then started a short feud with Diamond Dallas Page, to whom he lost in a Taped Fist match at Bash at the Beach. At World War 3, he unsuccessfully participated in a three ring 60-man battle royal to determine the #1 contender to the WCW World Heavyweight Championship. Duggan had no major pay-per-view matches for over two years before he disappeared in 1998.
Duggan returned and replaced Buff Bagwell in a match against Berlyn at Fall Brawl, which Duggan lost. Soon after, Duggan became involved in a storyline where he became a janitor for WCW. Duggan then started a feud with the anti-American stable the Revolution (Shane Douglas, Dean Malenko, Perry Saturn, and Asya). At Starrcade, he and his surprise partners, the newly reunited Varsity Club (Kevin Sullivan, Mike Rotunda, and Rick Steiner), faced the Revolution in a losing effort after the Club turned on Duggan. As a result of losing, the next night on Nitro, Duggan was forced to denounce the U.S. flag but he refused to do so. In response, the Revolution attacked him and began beating him until The Filthy Animals, who were also feuding with the Revolution, saved Duggan.
During this period, he found the World Television Championship in a trash can, which had been thrown away by the previous champion Scott Hall earlier and claimed it for himself. He would make title defenses exclusively on Saturday Night, and remained champion until the title was deactivated. After WCW was rebooted and the nWo storyline ended, it was replaced by two new factions in WCW called The Millionaire's Club and the New Blood, which began feuding with one another. The Millionaire's Club consisted of veterans and the New Blood consisted of young wrestlers of the 1990's. Duggan joined the Millionaire's Club and helped them in their war against the New Blood.
Near the end of his career in WCW, he became involved in more major storylines, including one that involved him becoming a villain by abandoning the patriotic gimmick for a short time to join Lance Storm and his faction Team Canada. This angle began when Duggan started working with the Misfits In Action (MIA), but at Fall Brawl, he turned on MIA's leader U.S. Champion General Rection during Rection's title defense against Team Canada's Lance Storm. Duggan was the special guest enforcer for the match, and helped Storm in winning the match. During this time Duggan shaved his trademark beard, cut his hair short, and sang "O Canada". At Halloween Havoc, Duggan and Storm teamed up against General Rection in a handicap match for Storm's U.S. Title. Duggan missed an attack on Rection, which led to Rection pinning Duggan to win the match. Duggan continued to unintentionally cost Team Canada many of their victories, but he helped Storm to defeat The Cat in a match at Starrcade. However, after the match, Team Canada turned on Duggan, who then began feuding with Team Canada, and at Sin he was the special guest referee in a Penalty Box match between Team Canada and the Filthy Animals. Despite Duggan's role as the referee, Team Canada won the match after Storm forced Kidman to submit.
Duggan appeared on the October 3, 2005 edition of Raw, dubbed "The Homecoming" along with several other WWF/E alumni. He and the other legends attacked Rob Conway after Conway came to the ring and insulted them. He was also one of the "legends" available for fans to select to team with Eugene in a match at Taboo Tuesday to face the team of Conway and Tyson Tomko (Jimmy Snuka was chosen instead of Duggan). He returned to WWE again after the Royal Rumble, with a message from John Cena to Lita after Edge lost the WWE Championship: "HOOOOO!" Duggan and Edge began a short feud with each other which culminated in a match that occurred two weeks later on Raw, which Edge won after Lita provided a distraction.
After feuding with Edge and Lita, he became mentor and tutor to Eugene and the two feuded with the Spirit Squad. He began working on Heat but also made a few appearances on Raw with Eugene and The Highlanders. On the November 6 edition of Raw, Duggan and Eugene faced the Spirit Squad in a match with the stipulation that the losing team would disband and never team up again. After Duggan was caught off guard after the other Spirit Squad members interfered and then pinned for the win, Eugene snapped and attacked Duggan. He started a feud with Eugene, and three weeks later, the two had a match with each other, which Eugene won.
At New Year's Revolution, Duggan teamed with Super Crazy in a tag team turmoil match to determine the number one contenders to the World Tag Team Championship, in a losing effort. On the edition of January 15 of Raw, Duggan was dragged out by Rated-RKO (Edge and Randy Orton) and was beaten to a pulp. Edge and Orton feared Duggan would help out Shawn Michaels in a handicap match and wanted to set an example for anyone else backstage that wished to do the same. On the March 5 edition of Raw, he faced Chris Masters in a Masterlock Challenge, but was unable to break the hold.
After months of wrestling on Heat, Duggan returned to Raw in July, teaming up with The Sandman and feuding with Carlito and William Regal. This team was short lived as Duggan soon returned to Heat and Sandman was soon released from his contract with WWE. On the August 6 Raw, Duggan was a contestant in the WWE Dating Game. However, Duggan lost when Maria chose Ron Simmons.
On December 31, he lost a 2008 Royal Rumble qualifying match to the returning Umaga. After that, he began primarily appearing on Heat once again, defeating locals and teaming with Super Crazy. On the March 17 episode of Raw, he participated in a 17-on-2 handicap elimination tag team match as part of the 17 members of the Raw roster against John Cena and Randy Orton. After Cody Rhodes was eliminated, all the remaining Raw wrestlers attacked Cena and Orton, resulting in their disqualification. Duggan also took part in a 12-on-12 tag team preview match on the March 25 episode of ECW. He competed in the 24-man battle royal that was held just before WrestleMania XXIV to determine the #1 contender for an ECW Championship match. The match was eventually won by Kane.
In the summer of 2008, Duggan started a short feud with Cody Rhodes and Ted DiBiase after Rhodes and DiBiase tried to convince Duggan to retire and let the young talent run the show. Soon after, Duggan would come out and announce that he would take Rhodes and DiBiase's advice and retire, but Jerry Lawler stopped him by convincing Duggan to stay. Duggan and Lawler then wrestled Rhodes and DiBiase on the August 25, 2008 edition of Raw, but lost the match. On the edition of October 20 of Raw, Duggan confronted Santino Marella and Beth Phoenix during one of Marella's tirades against his three possible opponents at Cyber Sunday (Roddy Piper, Goldust, and The Honky Tonk Man). Upset at being interrupted, Marella took off his clothes, getting ready to fight Duggan. Yet before a fight could begin, Marella instructed Phoenix to throw the clothes in Duggan's face, a move that temporarily distracted Duggan. Marella then smashed a guitar over Duggan's head, ending the segment. Duggan would appear on Raw's 800th episode during Kung Fu Naki's in-ring dance segment, which also involved various other members of the WWE roster.
In 2009, Duggan appeared in the 2009 Royal Rumble match, entering at #29 and setting a record for the most years between Royal Rumble appearances. He was eliminated from the match by Big Show. On the August 10 episode of Raw, Duggan made his final televised appearance as Sgt. Slaughter had him come out in place of Bret Hart, due to the episode taking place in Canada, when he introduced "The best there was, the best there is, and the best there ever will be" in order to mock the Canadian fans.
On April 2, 2011, Duggan was inducted into the WWE Hall of Fame by fellow inductee "The Million Dollar Man" Ted DiBiase.
Duggan returned to wrestling in WWE with an appearance at the 2012 Royal Rumble and entered the Royal Rumble match as the #19 entrant and was eliminated by Cody Rhodes. On the February 3, 2012 episode of SmackDown, Duggan teamed with Santino Marella in a losing effort against the WWE Tag Team Champions Primo and Epico. On April 10, 2012, Duggan made an appearance on SuperSmackDown LIVE: Blast from the Past, where he lost to Hunico by disqualification. Later that year, he appeared on the July 3 SuperSmackDown LIVE: The Great American Bash event, teaming with Sgt. Slaughter and Santino Marella to face Hunico, Camacho, and Drew McIntyre.
He defeated Jack Swagger by disqualification on March 4, 2013 on WWE Raw.