On the July 20, 1991 episode of WWF Superstars of Wrestling, Sid made his World Wrestling Federation debut as a babyface under the name Sid Justice. He was announced as the special guest referee for the main event of that year's SummerSlam, where The Ultimate Warrior and then-WWF Champion Hulk Hogan teamed up against The Triangle of Terror (Sgt. Slaughter, Col. Mustafa, and Gen. Adnan) in a 3-on-2 handicap match. Later that night, Sid saved "The Macho Man" Randy Savage and Miss Elizabeth from an attack at the hands of The Undertaker and Jake "The Snake" Roberts at the newly wed couple's reception. Sid dominated a short feud against The Undertaker before he took on Roberts. In a match with Roberts, Sid injured his biceps and was forced to miss the Survivor Series pay-per-view.
Sid returned to active duty at the Royal Rumble, emanating from Albany New York that year. This particular edition had a special stipulation: the winner would win the vacant WWF Title, which had been stripped from Hulk Hogan after two controversial title switches in a row between Hulk and the Undertaker. Sid entered at #29, and was among the final four wrestlers, along with Hulk Hogan, Randy Savage, and Ric Flair, before he eliminated both Savage and then Hogan, leaving himself and Flair in the ring. During the initial live pay-per-view broadcast, this action was loudly cheered by the audience in attendance even though, as per storyline plans, Sid "sneaked up from behind" to throw Hogan out. The reaction is in retrospect widely believed to be due to a combination of fans tiring of Hogan's all-American superhero persona, and also responding positively to Sid's charisma. Nevertheless, this reaction was edited out of future television replays as well as the Coliseum home video release of the event; Play-by-play announcer Gorilla Monsoon even re-recorded his voiceover commentary, now condemning Sid for his actions when he previously said that Justice's elimination of Hogan was fair. Hogan, who was still at ringside after being eliminated, grabbed Sid's arm and distracted him long enough for Flair to eliminate him to win the match and become the new WWF Champion. After the match, Sid and Hogan got into an argument in the ring and had to be separated by security. This incident began Sid's turn into a villain.
Less than a week later, on the January 25 episode of Superstars, WWF President Jack Tunney held a press conference to announce who among Sid, Savage, Hogan, "Rowdy" Roddy Piper and The Undertaker would face Ric Flair for the WWF Championship at WrestleMania VIII. Before Tunney even announced who the number one contender would be, Sid stood up as if Tunney called his name. Yet to Sid's annoyance, Tunney chose Hogan. After the press conference, Sid referred to it as "the most bogus act Jack Tunney has ever pulled off." Sid later issued an apology to Hogan, which Hogan accepted. Sid and Hogan then teamed up to face The Undertaker and Flair on the February 8 edition of Saturday Night's Main Event. During the match, after he double clotheslined Undertaker and Flair, Hogan reached to Sid for a tag. However, Sid refused to tag in and walked out of the match, completing his heel turn. Despite this, Hogan won the match by disqualification.
On February 23 on an episode of Wrestling Challenge, Sid appeared as a guest on Brutus "The Barber" Beefcake's "The Barber Shop". Knowing that Hulk Hogan (Beefcake's good friend) was not in the arena, Sid attacked Beefcake and destroyed the Barber Shop set with a chair. Later that night, it was announced that Hogan would battle Sid (and not WWF Champion Ric Flair) at the main event of WrestleMania VIII, resulting in Flair facing Randy Savage for the WWF Title instead. A week later, Sid hired Harvey Whippleman as his manager. Sid also began a post-match gimmick where he would further "injure" his defeated opponents with one or more powerbombs (his finishing move), and sometimes — after the defeated wrestler placed on a stretcher — following this up by grabbing the stretcher and running it into a fixture, such as a ringpost or guardrail.
At WrestleMania VIII, Sid lost his match to Hogan by disqualification when Papa Shango interfered on Sid's behalf, allowing the two to double-team Hogan until the returning Ultimate Warrior stormed the ring and saved Hogan. During the match, Sid kicked out of Hogan's trademark Atomic Leg Drop; according to popular belief, Charles Wright (who portrayed Papa Shango) missed his cue to break up the pin attempt. During the match, Monsoon referred to Sid as "Sycho Sid," a name that Sid would use during his later-1990s run in the WWF.
On the February 20, 1995 episode of Monday Night Raw, Sid returned to the WWF under the ring name Sycho Sid, where he became the bodyguard of Shawn Michaels. He, along with Jenny McCarthy, accompanied Michaels to ringside for Michaels' WWF Championship match against the champion and Michaels' former bodyguard, Diesel, at WrestleMania XI. Michaels had the match won after hitting his signature Sweet Chin Music, but Sid stood on the ring apron and distracted referee Earl Hebner, allowing Diesel time to recover and pin Michaels after a Jackknife Powerbomb to win the match and retain his title. The next night on Raw, Michaels expressed dissatisfaction with Sid's interference and gave him the night off for his rematch against Diesel at the first-ever In Your House pay-per-view. In response, Sid replied to Shawn, "You don't give me the night off!", and attacked Michaels from behind and powerbombed him three times, turning Michaels into a face again. Diesel came to Michaels' aid and clotheslined Sid over the top rope. Michaels sustained a legitimate back injury as a result of the attack and was sidelined for six weeks, thus taking him out of the title bout.
Two weeks later on the April 17 episode of Raw, "The Million Dollar Man" Ted DiBiase announced Sid as the newest member of the Million Dollar Corporation after Bam Bam Bigelow left the Corporation. After joining, Sid challenged Diesel to a match for the WWF Championship at In Your House, which Diesel accepted. Diesel won the match via disqualification, and thus retained his title, when Tatanka interfered. After the match, Sid and Tatanka continued to double-team Diesel until Bam Bam Bigelow came out to save him. At the King of the Ring, Diesel and Bam Bam Bigelow defeated Sid and Tatanka. Sid faced Diesel once again at In Your House 2: The Lumberjacks for the WWF Title in a lumberjack match, which Diesel won to end the feud. Following this, Sid onto a feud with Shawn Michaels and was scheduled to face him at SummerSlam, but was replaced by Razor Ramon at the request of WWF President Gorilla Monsoon, with Ramon challenging for Michaels' Intercontinental Championship in a ladder match as Sid was seen watching on the backstage television monitors. On the September 5 episode of Raw, Sid faced Michaels for the title but lost after being hit with Sweet Chin Music three times. Sid then started a brief feud with Henry Godwinn, culminating in a victory over Godwinn at In Your House 3: Triple Header.
On the November 13 episode of Raw, Sid faced the Intercontinental Champion Razor Ramon in a non-title match, with Ramon's friend The 1-2-3 Kid as the special guest referee. Razor was about deliver the Razor's Edge on Sid, but The 1-2-3 Kid helped Sid avoid it, allowing Sid to pin Ramon with a powerbomb after with the Kid making a fast count. After the match, the Kid turned heel and joined the Million Dollar Corporation. In the first elimination match at Survivor Series, Sid and Corporation leader Ted DiBiase helped The 1-2-3 Kid pin Marty Jannetty to win and become the sole survivor for his team. Later in the event, Sid was randomly teamed up with his rival Shawn Michaels, Ahmed Johnson, and The British Bulldog to face Yokozuna, Owen Hart, Razor Ramon, and Dean Douglas in a "Wild Card" Survivor Series Match. Sid was eliminated by Razor Ramon after Michaels hit Sid with Sweet Chin Music. After his elimination, Sid powerbombed Michaels. At In Your House 5: Season's Beatings, Razor Ramon and Marty Jannetty defeated Sid and The 1-2-3 Kid. Sid and The 1-2-3 Kid teamed up the next night to participate in the first-ever Raw Bowl, which The Smoking Gunns won.
A severe injury sustained before or during a January 2, 1996 house show match in Hartford, Connecticut with Bob Holly led to Sid being taken off television.
Sid would not be seen again until the July 8 episode of Raw, when he was announced as the replacement of The Ultimate Warrior (who had left the WWF) for the six-man tag team match, teaming with former rival Shawn Michaels and Ahmed Johnson against Vader, Owen Hart, and The British Bulldog at the main event of In Your House 9: International Incident, effectively making him a face. However, Sid's team lost the match.
The next night on Raw, Sid started a feud with The British Bulldog. Sid faced the Bulldog at SummerSlam on August 18 and pinned Bulldog with the powerbomb, despite interference from Owen Hart. At In Your House 10: Mind Games on September 22, Shawn Michaels hit Mankind with Sweet Chin Music and went for the pin to retain the WWF Championship, but Vader came out, broke up the count, attacked Michaels, and got Mankind disqualified. After the match, Mankind and Vader double-teamed Michaels until Sid came out to make the save. He and Vader fought their way backstage, starting a feud between the two.
Sid fought Vader at In Your House 11: Buried Alive on October 20 in a match where the winner would face Shawn Michaels for the WWF Championship at Survivor Series on November 17. As Sid was about to powerbomb Vader, Vader's manager Jim Cornette got on the ring apron to distract him. Michaels responded and pulled Cornette off the apron before hitting him with Sweet Chin Music. Sid then pinned Vader with a chokeslam to win the match and to become the number one contender for the WWF Championship. After the match, Sid celebrated his victory with Michaels.
At the 1996 edition of the Survivor Series, history repeated itself. Sid grabbed a camera from the cameraman and prepared to hit Michaels with it. Michaels' manager, Jose Lothario, got on the ring apron and told Sid to put the camera down, but Sid refused and hit Lothario in the chest with the camera instead. Although this was the act of a heel, the audience cheered wildly for him and booed Michaels, just as they had done, in Sid's favor, four and a half years earlier against Hogan at the Royal Rumble. Sid dropped the camera and as soon as he turned around, Michaels hit him with Sweet Chin Music; however, Michaels went outside the ring to check on his manager instead of going for the pin. Sid hit Michaels in the back with the camera, then threw him back in the ring before hitting him with the powerbomb to win the WWF Championship.
At In Your House 12: It's Time on December 15, Sid defended the title against Bret Hart in a match where the winner would defend the title against Shawn Michaels at the 1997 Royal Rumble on January 19. Hart made Sid tap out to the Sharpshooter, but the referee was knocked out and unable to witness the submission. As Shawn Michaels was commentating at ringside, Sid and Hart left the ring and started fighting right beside him. After Sid pushed Michaels then climbed into the ring with Hart, Michaels went to hit Sid but Sid threw Hart into him. Sid then pinned Hart with the powerbomb to retain the title.
At the Royal Rumble, Sid defended the title against Michaels. During the match, Sid hit the chokeslam on Michaels and repeatedly powerbombed him outside the ring. Later on in the match, Jose Lothario got on the ring apron and Sid approached him, but before he could do anything to Lothario, Michaels hit Sid in the back and the face with the camera, knocking him out in the process. Michaels went for the pin, but Sid managed to kick out. Michaels then hit Sid with Sweet Chin Music to become the WWF Champion for the second time. Michaels soon forfeited the title due to being unable to wrestle, a claim still widely disputed by many during that time, especially Bret Hart, who believe Michaels didn't want to drop the title to him at Wrestlemania XIII. At In Your House: Final Four in Chattanooga, a four-way title match was held for the vacant belt between Bret Hart, the Undertaker, Vader, and Stone Cold Steve Austin. Hart won, and was scheduled to face Sid the following night, on the February 17, 1997 episode of Raw. During that match, Hart had Sid trapped in the Sharpshooter submission when Stone Cold Steve Austin, whom Hart was feuding with, came to the outside of the ring and hit Hart with a steel chair, allowing Sid to hit Hart with the powerbomb to win the WWF Championship for the second time.
At WrestleMania 13 on March 23, Sid defended the title against The Undertaker. Hart interfered during the match, allowing The Undertaker to hit the Tombstone Piledriver and pin Sid to become the WWF Champion for the second time. Following WrestleMania, Sid was kept off of television until the May 12 episode of Raw, where he returned as a face. He competed against Owen Hart and defeated him to become the partner of The Legion Of Doom in their feud with The Hart Foundation. At King of the Ring on June 8, Sid and The Legion Of Doom faced The Hart Foundation (Owen Hart, The British Bulldog, and Jim Neidhart) in a six-man tag team match, which The Hart Foundation won when Owen pinned Sid with a roll-up. On June 9, Sid made his final appearance in a victorious effort against Owen Hart on Raw before leaving the WWF once again.
On June 25, 2012, Sid returned to Monday Night RAW for its 996th episode in a match against "The One Man Southern Rock Band" Heath Slater, in which Sid defeated Slater in celebration building up to WWE's 1,000th episode of RAW. It was his first appearance on RAW since July 14, 1997 and his first match on RAW since June 9, 1997.