Trinidad Sr. began his professional boxer career by taking on 9-1 Nick Ortiz on March 31, 1975, in San Juan, losing by sixth-round knockout. Three weeks later, he got his first professional boxing win when he defeated a debutant, Francisco Sullivan, by a second-round knockout, also in San Juan. Trinidad Sr. lost two of his next three bouts and took time off boxing from 1976 to 1978, when he returned on January 14 with a victory over 1-2 Fernando Rivera by six rounds decision at
Carolina, Puerto Rico. Trinidad Sr. followed that win with three more victories and then he was given a chance at winning the vacant Puerto Rican national featherweight title on November 27 of the same year-1978-as part of the undercard in which Solis lost to
Eusebio Pedroza at the
Roberto Clemente Coliseum in San Juan for Pedroza's
WBA world Featherweight title. His rival that night, the same Fernando Rivera that Trinidad Sr. had previously defeated, was by now 4–3 in seven bouts. Trinidad Sr. won the Puerto Rican national Featherweight title by outpointing Rivera again, this time over twelve rounds. Next, Trinidad Sr. had his first fight abroad, against up-and-comer, future
WBC world Featherweight champion and International Boxing Hall of Fame member, 31-1-1 Salvador Sanchez. This bout took place on August 7, 1979, at
The Summit in
Houston,
Texas, United States, and Trinidad Sr. lasted until the fifth round, when he was stopped by the young Mexican. A loss to Enrique Solis on December 18 by ninth-round knockout followed, then two more losses, one to future world title challenger, 14-1
Carlos Pinango by a knockout in round eight at
Caracas,
Venezuela on August 18, 1980, before Trinidad Sr. posted what would turn out to be his last win as a professional boxer, beating 7-2 Fernando Ortiz on January 29, 1981. On February 19, 1981, Trinidad Sr. tried to regain his by now vacant national Featherweight championship against debuting Dagoberto Agosto. In what would turn out to be his last fight, he lost to Agosto by a twelve-round decision. Trinidad Sr. retired with a record of 8 wins and 8 losses in 16 professional boxing bouts, 5 wins coming by way of knockout. ==Training career==