He started his career as a professional boxer with a loss, being defeated by decision in eight rounds by Juan Jose Jimenez, October 23 of 1959 in Santo Domingo. His first win came on December 3 of that year, also in Santo Domingo, with a ten round decision win over Rafael Acevedo.
After one more win in Santo Domingo, he moved to San Juan, Puerto Rico. There, he posted a record of 7 wins and 6 losses before returning to Santo Domingo in 1962. Out of the 7 wins in Puerto Rico during that era, 5 were by knockout. In his return to Santo Domingo, he posted a decision win over Acevedo in a rematch. Towards the end of 1962, he started campaigning in the United States, particularly in New York. There, he boxed 5 times before returning to San Juan for another bout. He won 4 and drew 1 of those fights, all wins by decision.
He spent the first half of 1964 touring Australia, where he won 2 fights and lost one. Then, he returned to Latin America, his first fight after arriving in Australia being a major step up in quality of opposition for him: In Caracas, he met fellow world champion boxer Carlos Morocho Hernandez. He was knocked out in two rounds by Hernandez. On to Panama City, where he lost a ten round decision to Julio Ruiz. He finished his year by beating Marcos Morales in Santo Domingo.
He won 4 bouts, lost 1 and drew 1 in 1966. He drew with Jaime Valladares in Quito, and lost to Frankie Narvaez in San Juan. But he also beat former world title challenger Bunny Grant. In 1967, he avenged his loss to Narvaez, and went undefeated the rest of the year, securing his position as the world's number one challenger among Lightweights.
He won two more fights to begin 1968, and then, on June 29 in Santo Domingo, he was given his first chance to challenge for a world title. He became world Lightweight champion when he defeated Carlos Ortiz by a decision in fifteen rounds.
There was a rematch between Cruz and Ramos, also held in Los Angeles. The second time around, Ramos became world Lightweight champion by beating Cruz with a nine round knockout. Cruz went on to win his next three bouts of 1969.
On January 17 of 1970, Cruz won what would turn out to be, tragically, his last fight. He beat Benito Juarez in San Juan by a decision in ten, and then returned to Santo Domingo.
On February 15, he was flying back to San Juan alongside his family for a rematch with Ortiz, when their Dominicana de Aviacion DC-9 plane crashed into the waters of the Caribbean Sea shortly after take-off, killing Cruz, his wife and kids, and the rest of the passengers, among which also were a large part of Puerto Rico's national volleyball team (see: Dominicana DC-9 air disaster)
Cruz had a record of 41 wins, 14 losses and 2 draws as a professional boxer, 13 wins coming by knockout.
Cruz's younger brother, Leo Cruz, later on went on to become a world champion too.