Stamboli debuted on WCW television in late 1999. He adopted the ring name Johnny the Bull and formed a Mafia-themed tag team called The Mamalukes with Big Vito. The duo were managed by Tony Marinara.
On January 18, 2000 in Evansville, Indiana, the Bull and Big Vito defeated Crowbar and David Flair for the WCW World Tag Team Championship. The match was broadcast on the January 19 episode of Thunder. They held the championship until February 12, when they were defeated by Creative Control in Oberhausen during a tour of Germany. The Mamalukes regained the championship on the following day in Leipzig, and held it until March 19, when they lost to Creative Control in a no disqualification match at the pay-per-view Uncensored, which took place in Miami, Florida.
On June 5, 2000, The Mamalukes attacked Hardcore Champion Terry Funk during a title defense against WCW President Eric Bischoff, enabling Bischoff to defeat Funk. On the June 7 episode of Thunder, Bischoff awarded the Hardcore Championship to The Mamalukes, making them co-champions. On June 19, Big Vito defeated Johnny the Bull to become the undisputed Hardcore Champion. On the July 3 episode of Nitro, while wrestling Funk in a hardcore match, Johnny tore his urethra while executing a diving leg drop and was sidelined for two months while recuperating.
On the September 11, 2000 edition of Nitro, he returned to betray Vito and joined a stable known as The Natural Born Thrillers, a collection of young wrestlers who sought to challenge more established wrestlers. Although the Thrillers were successful in winning the World Tag Team Championship on several occasions, Johnny reconciled with Vito and reformed the Mamalukes, and the two continued teaming until WCW's closure in March 2001.
In March 2001, World Championship Wrestling was purchased by the World Wrestling Federation. Johnny the Bull's contract was bought out by the WWF. Johnny the Bull wrestled one dark match before the July 12, 2001 edition of SmackDown!, losing to Rob Van Dam. Johnny the Bull was introduced to the promotion as a member of The Alliance but after a single appearance on WWF television, he was taken off of television.
After the company was renamed to World Wrestling Entertainment in May 2002, he returned to WWE television on the June 30, 2002 episode of Sunday Night Heat under the ring name Johnny "The Bull" Stamboli, losing via disqualification to Tommy Dreamer. On the July 15 episode of Raw, Stamboli won the Hardcore Championship, his first title in WWE, from Bradshaw. Bradshaw would, however, regain the title from Stamboli later that night. On the July 29 episode of Raw, Stamboli won the Hardcore Title once again after pinning Jeff Hardy, but lost the title a few seconds later to Tommy Dreamer. Following this, he became a mainstay on Heat and would later turn face due to fan reaction in late September 2002.
On February 13, 2003, Stamboli was moved to the SmackDown! brand and became a heel again when he allied with Nunzio and Chuck Palumbo to attack Rikishi. Together, the trio became The Full Blooded Italians. The FBI would go on to wrestle throughout 2003 and early 2004 before disbanding on March 22, 2004 when Palumbo was moved to the Raw brand. Stamboli and Nunzio continued to team together until he was released from his WWE contract on November 4, 2004.