When Gunn first arrived in the World Wrestling Federation, he teamed with his on-screen brother, Bart Gunn, as The Smoking Gunns. In early 1995, they won their first Tag Team Championship by defeating the makeshift team of Bob Holly and 1,2,3, Kid. They held the titles until WrestleMania XI when 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 Title 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 Title 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 briefly feuding with his brother, Gunn adopted a new gimmick, Rockabilly, and became the protégé of The Honky Tonk Man. During this time, he 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. Although initially a heel tag team, Gunn turning on the Honky Tonk Man actually got a cheer from the live crowd.
James and Rockabilly were quickly rebranded as "Road Dogg" Jesse James and "Badd Ass" Billy Gunn, respectively, and their tag team formed was soon named the New Age Outlaws. This new team was in the spirit of the new Attitude Era: brash, vulgar, egotistical, and loud-mouthed. 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, who were impressed with their new attitude. 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). Upon becoming an official member of DX, Gunn picked up the tendency of mooning opponents (a trait used by the original incarnation of DX, as well as its 2006 revival).
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. The Outlaws would also successfully defend their titles against The New Midnight Express (Bombastic Bob and Bodacious Bart) at King of the Ring.
During this time, the Outlaws began a feud with Kane and Mankind. Although Kane and Mankind won the Tag Team championship, they could not get along. 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. The Outlaws then helped X-Pac in his feud with Jeff Jarrett and Southern Justice. In December, the Outlaws lost their title to The Big Boss Man and Ken Shamrock from 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 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 feud with The Rock. At SummerSlam, The Rock defeated Gunn.
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. After defeating teams such as Edge and Christian, Hardcore Holly and Crash Holly, and The Acolytes, the Outlaws reunited with X-Pac and Triple H to reform D-Generation X. The group then feuded with the likes of Steve Austin, The Rock, Kane, Mankind, Shane McMahon, and Vince McMahon. During this time, The Outlaws won their fifth and final 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 heated 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". He then took time off to tend to his arm injury, before returning later in the year.
Gunn made his return in October and immediately teamed with Chyna to feud with Right to Censor. At No Mercy, Right to Censor members Steven Richards and Val Venis defeated Chyna and 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 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. Gunn competed for the Hardcore Championship until June, when he turned heel and had a short feud with the 2001 King of the Ring, Edge. After the feud, Gunn quietly turned face again and formed a short lived tag team with Big Show. At Show's request, the team was called "The Show Gunns", a pun on the word shogun. At InVasion, The Show Gunns and Albert lost to the team of Alliance members Shawn Stasiak, Hugh Morrus, and Chris Kanyon. The Show Gunns quietly disbanded soon after and Gunn began feuding with mid-card members of The Alliance.
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 face tandem, but soon turned heel when they were given a gimmick where they grew increasingly affectionate toward each other.
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 defeating The APA, The Dudley Boyz, and The Hardy Boyz in a Four Corners Elimination Match at WrestleMania X8, and Al Snow and Maven at Backlash, Billy and Chuck feuded with Rikishi. At Judgment Day, Rikishi and Rico (Rikishi's mystery partner of Mr. McMahon's choosing) defeated Billy and Chuck for the Tag Titles after Rico accidentally hit Chuck with a roundhouse kick. Billy and Chuck quickly won the titles back two weeks later on SmackDown! with Rico's help. They held the championship for about a month before losing it to the team of Edge and Hulk Hogan.
Later that summer, 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 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.
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, effectively turning Billy and Chuck face in the process. At Unforgiven, Three Minute Warning defeated Billy Gunn and Chuck Palumbo. 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, Sopp 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, he added Torrie Wilson as his 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. On SmackDown!, it looked as if Torrie was going to have a Ménage à trois with Noble and Nidia until Gunn broke into the hotel room ready to brawl with Noble. Billy then started a short lived tag team with Noble before taking time off again due to a shoulder injury.
Gunn returned to action at the 2004 Royal Rumble, but was eliminated by Goldberg. He also competed in a battle royal for a WWE Championship match at No Way Out, but was eliminated by Eddie Guerrero, who went on to win the match. Afterward, he wrestled mainly on Velocity forming a tag team with Hardcore Holly. At Judgment Day, they unsuccessfully challenged Charlie Haas and Rico for the Tag Team Championship. Sopp then had a short feud with Kenzo Suzuki before beginning a pursuit for the United States Championship.
On November 1, 2004, Gunn was released from his WWE contract. At the time of his release, he was one of the most tenured wrestlers with the company, behind only The Undertaker and Shawn Michaels.
On July 23, Billy Gunn made his first WWE appearance in nearly eight years as he reunited with Road Dogg, X-Pac, Triple H, and Shawn Michaels to reform D-Generation X for one night only on the 1000th episode of Raw.
In December 2012, Billy was hired as a trainer by WWE and assigned to the NXT Wrestling territory in Tampa, Florida. On the December 17 episode of Raw, Gunn and Road Dogg reformed the New Age Outlaws for one night only as they presented the Slammy Award for Comeback of the Year to Jerry Lawler. The New Age Outlaws appeared at the December 27th WWE Live Events at Madison Square Garden and on the 28th at the Nassau Coliseum. On March 4, 2013, Billy 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 F5. At an NXT show Gunn refereed an NXT Championship match between Bo Dallas and Sami Zayn and allowed Bo Dallas to win and retain the NXT Championship. He later explained what he did by saying Triple H asked him to do it and it was what's best for business. NXT GM JBL later showed his support for what Gunn did. 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 11 episode of SmackDown, the Outlaws would team with CM Punk in a six-man tag match against The Shield in a losing effort. On the January 13, 2014 episode of Raw, the Outlaws would once again team with Punk in a rematch against The Shield, only to abandon Punk and lose the match, turning heel in the process for the first time in over a decade.
On January 26th, 2014 during the Royal Rumble Kickoff Show, Gunn and Road Dogg as the New Age Outlaws beat Cody Rhodes & Goldust to win the WWE Tag Team Championship. This marks the Gunn's first reign with this particular title, and his 11th overall tag team championship reign in WWE. The next night on Monday Night Raw the New Age Outlaws retained their titles against Cody Rhodes & Goldust via disqualification when Brock Lesnar attacked the brothers. The next week on Raw the New Age Outlaws put their titles on the line against Cody Rhodes and Goldust in a steel cage match where Gunn picked up the victory by doing the Famouser on Cody Rhodes after Rhodes hit Road Dogg with a moonsault from the cage. After successfully defending the titles at Elimination Chamber 2014 against The Usos, they lost the belts to the twins on the March 3 Raw. Billy Gunn sustained hemoptysis after he and his New Age Outlaws partner, Road Dogg, suffered a double-Triple Powerbomb by The Shield at WrestleMania 30. This was also used to write both Gunn and Road Dogg off television.
Gunn returned to Raw with Road Dogg in January 2015, attacking The Ascension along with the nWo and the APA. At Royal Rumble, the Outlaws faced The Ascension in a losing effort. At WrestleMania 31, Gunn, with Road Dogg, Sean Waltman 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, Sopp was released from WWE after failing a test for performance-enhancing drugs.