Mercado started boxing as an amateur at the age of 12, immediately winning a nationwide competition known as the "Olimpiadas Jíbaras de la Vivienda" ("Housing Complex Townspeoples' Olympics"). This motivated him to keep boxing as an amateur and hoping to win a world title in the future. In 1978, Mercado represented Puerto Rico at the
1978 Central American and Caribbean Games held at
Colombia. He won gold at these games and participated in a world cup tournament. Hoping to become the first Puerto Rican ever to win a gold medal at an Olympic Games, Mercado moved to
Cuba periodically; there he trained hard towards that goal. Around that era, Mercado befriended and worked alongside one of
Jose Celso Barbosa's children; he criticized the Puerto Rican government which was led by
Carlos Romero Barcelo of the Statehood supporting
New Progressive Party and was fired from his job and taken by Puerto Rico Olympic Committee President
German Rieckehoff Sampayo to live in
Mexico for a while. Mercado was one of only three
American citizens to participate in the
1980 Olympics celebrated in
Moscow,
Soviet Union, bearing the flag of and competing in boxing for
Puerto Rico after having won the gold medal at the
1979 Pan American Games. The other two were also representants from Puerto Rico and boxers:
Luis Pizarro and
José Angel Molina. During a 2021 Interview with Cuban-Puerto Rican journalist Jesus "Chu" Garcia on
YouTube, Mercado remembered how he and his fellow Puerto Rican boxers Pizarro and Molina were the first delegates of any kind to arrive in Russia and how he flew on
Aeroflot from Cuba to Russia to get there. ==1980 Olympic results==