At age 16, Lopez was signed by the Phillies in 1959, and was mostly a starting pitcher for 5 seasons in their farm system, including 2 in triple-A, before regressing to double-A in 1964. Marcelino also played in 4 games for the Phillies in April 1963, before returning to the minors.
In October 1964, Lopez was sent to the Angels as the player to be named later in the deal that brought rent-a-player Vic Power to the Phils for their pennant push that season.
Lopez was one of the Angels' top 3 starters during 1965 and 1966, He started 32 games each season, and was one of 3 Angels' pitchers to win at least 14 wins in 1965 (along with Dean Chance and Fred Newman). Marcelino finished 2nd to Curt Blefary in the 1965 Rookie of the Year award.
His record slipped to 7-14 in 1966, and shoulder problems in 1967 limited him to 4 games with the Angels before June 15th, when he was dealt to the Orioles for veteran utility man Woodie Held. He played 4 games with the O's, but spent most of 1967 and all of 1968 in the minors.
Lopez pitched 2 seasons in the Orioles bullpen, and appeared in the post-season in '69 and '70. Traded to the Brewers the following spring, he switched back to the starting rotation during the 1971 season, his last full season in the majors.
In March 1972, Lopez was sold to the Indians, but he spent most of that season in the minors. Marcelino played in the minors from 1974-76, including a stint in the Mexican League during 1974.
He passed away in November 2001 at age 58.