In 1978, Estrada found himself working for the World Wide Wrestling Federation (WWWF) as Carlos José Estrada, sometimes billed simply as Carlos Estrada or José Estrada. On January 20, 1978, Estrada defeated Tony Garea in a tournament final to win the WWWF Junior Heavyweight Championship, a championship that had been inactive for at least six years prior to the tournament. Three days later, Estrada dropped the championship to Tatsumi Fujinami.
In 1987, he returned to the World Wrestling Federation, where he adopted a new masked persona, known as Conquistador Dos, teaming with José Luis Rivera as Conquistador Uno, to form a team known as Los Conquistadores. The team was used mainly as enhancement talent. Los Conquistadores participated in the 1988 Survivor Series. At the Survivor Series they were part of a special elimination match, teaming with Demolition (Ax and Smash), The Brain Busters (Arn Anderson and Tully Blanchard), The Bolsheviks (Nikolai Volkoff and Boris Zhukov), The Fabulous Rougeaus (Raymond and Jacques Rougeau). The five tag teams took on the collective face team of The Powers of Pain (The Warlord and The Barbarian), The Rockers (Shawn Michaels and Marty Jannetty), The British Bulldogs (Davey Boy Smith and Dynamite Kid), The Hart Foundation (Bret Hart and Jim Neidhart) and The Young Stallions (Jim Powers and Paul Roma). The match came down to Los Conquistadores and the Powers of Pain as the last team on each side, with The Powers of Pain winning after Demolition's manager Mr. Fuji switched sides and helped the Powers of Pain win the match. This match would make the only time Los Conquistadores would wrestle on a WWF pay-per-view show. By 1989, Estrada worked in singles matches, both as "Conquistador Dos" and under his real name.