Mercedes campaigned most of his career in the western United States and in Mexico. He debuted as a professional boxer on July 2 of 1978 with a win over Darryl Jones in Stockton. He suffered his first defeat when he lost by a decision in Las Vegas to future world champion Joey Olivo, in Mercedes' third bout as a paid fighter.
Mercedes' first of 11 fights in a row in Mexico came in 1979, when he lost by a knockout in seven rounds to Juan Diaz. During that span of 11 fights in Mexico, he went 6-3-2, fighting in such places as Mexico City, Monterrey and Reynosa. He lost to Candido Tellez but also beat former world champion Guty Espadas, by a knockout in nine during that span.
His first fight in the Dominican Republic was a 1982 rematch with Diaz, held in Santo Domingo. He won the fight by a decision in twelve rounds, to avenge his defeat at the hands of Diaz. Up until that moment, Mercedes' record was of 10 wins, 6 losses and 2 draws, reason why it was a little surprising to his fans when the WBC announced it would give Mercedes a chance at becoming a world champion, against two time world champion Freddie Castillo, who was in his second reign. The two boxers met on November 6 of 1982, at Los Angeles. Mercedes caused another surprise among many boxing fans by defeating Castillo by a decision in 15 rounds and becoming world Flyweight champion.
In his first defense, Mercedes put the title on the line against the British and Tunisian Charlie Magri, British by birth but Tunisian by mother and father. In round seven, the fight had to be stopped because of a cut suffered by Mercedes, which had been determined to be from a punch by Magri. Mercedes then lost his world Flyweight championship by a technical knockout in that round.
After that, he kept on boxing, but was only able to win some and lose some for the rest of his career. He beat future world title challenger Alonzo Gonzalez by a decision, but lost to future world title challenger German Torres by a knockout in nine, to future world champion Raul Jibaro Perez by a decision in twelve, and to former world Bantamweight champion Alberto Davila by a knockout in six.
A few days before Christmas in 1985, Mercedes was shot and killed by a policeman in his native La Ramon. In what could be called unclear circumstances, he was shot after allegedly pulling out a gun at the policeman. Mercedes had been drinking in a bar when he was shot to death.
He had a record of 14 wins, 12 losses and 2 draws, with 3 wins by knockout.