Guerrera debuted in Extreme Championship Wrestling alongside fellow luchadores Rey Misterio Jr., Psicosis, Konnan and La Parka in 1996. When Konnan left for WCW, Guerrera and the rest followed.
In WCW, Guererra made his debut on August 26, 1996 edition of Monday Nitro in a victory over Billy Kidman. He would compete throughout 1996 and 1997 under a mask, like the one he wore in ECW. His most remembered matches of 1996 and 1997 were clashes with fellow masked luchadores Rey Misterio Jr. and Psicosis.
Beginning in 1998, Guerrera pinned Último Dragón to win the Cruiserweight Championship on the inaugural edition of Thunder on January 8, 1998. However, he would then lose the title to Rey Misterio Jr. one week later on the following edition of Thunder. At SuperBrawl VIII, Guerrera challenged Chris Jericho for the title in a mask versus title match, in which Guerrera lost forcing him to unmask. After unmasking Guerrera, Jericho would often berate Guerrera for his looks by referring to him as "Quasi-juice" which was in reference to "Quasimodo". Later in the year, Guerrera defeated Jericho at Road Wild to capture his second Cruiserweight Championship. He retained the belt at Fall Brawl against Silver King, before losing to Billy Kidman the following night on Nitro.
Later on, Guerrera became an announcer on Thunder in order to remain on-screen while rehabilitating an injury. On November 29, 1999, Guerrera defeated Jushin Thunder Liger on Nitro. The next week, he would lose the title back to Liger, as Psicosis substituted for Guerrera due to injury. After his injury healed, he joined former rival Rey Mysterio Jr., Konnan, Billy Kidman, and others to form The Filthy Animals. During the group's run, Guerrera and Misterio Jr. became the World Tag Team Champions.
Guerrera was released by WCW in late 2000.
In the spring of 2005, Guerrera had signed a contract with World Wrestling Entertainment (WWE). He debuted on television, wrestling simply as Juventud, on June 18, 2005, defeating Funaki on SmackDown!'s sister show Velocity. On the June 23 episode of SmackDown, Juventud, Super Crazy and Psicosis (collectively called "The Mexicools") interrupted a match between Chavo Guerrero and Paul London by riding to the ring on a lawn mower and attacking both wrestlers. The following week by week of The Mexicools interrupting matches and beating down seemingly random wrestlers and cutting promos. Juventud quickly established himself as the mouthpiece/leader of the group and began using his old nickname "The Juice".
At the Great American Bash on July 24, The Mexicools defeated the newly reunited Blue World Order (Big Stevie Cool, Da Blue Guy, and Hollywood Nova) in a six-man tag team match. After more weeks of interruptions and sneak attacks, The Mexicools were sent down to Velocity. The group feuded with Velocity main-stay and then-Cruiserweight Champion Nunzio and his tag partner Vito. Each member of The Mexicools picked up numerous pinfall victories over the champion during this time.
At the October 4 SmackDown! tapings for Velocity, Juventud (along with Super Crazy and Psicosis) faced Brian Kendrick, Paul London, Scotty 2 Hotty, and Funaki in an over-the-top battle royal with the stipulation that the winner would receive a title shot at the Cruiserweight Championship at No Mercy. After finally eliminating London, Juventud won the battle royal and the title shot. At the event on October 9, he defeated Nunzio following a Juvi Driver to capture the Cruiserweight Championship, his first title in WWE. Juventud later went on to lose the title back to Nunzio at a live event in Italy. On November 25, Juventud regained the title on an edition of SmackDown! from England. Guerrera lost the Cruiserweight title to Kid Kash at Armageddon on December 18, 2005.
On January 6, 2006, WWE announced that Guerrera was released from the company. His final match aired that evening on SmackDown!, a Cruiserweight Title rematch against Kid Kash which Juventud lost.