Mariano Duncan had a resounding rookie season in 1985. Climbing
from Class AA to the Los Angeles Dodgers on Opening Day due to injuries in the Dodgers organization, and was third in NL Rookie of the Year balloting. He was the LA’s second baseman on Opening Day, and was moved into the role as the regular shortstop on June 1 due to an injury to Dave Anderson.
His all-around play as a rookie helped the Dodgers win the NL West. Mariano Duncan hit .244 with 24 doubles, six triples, six home runs, 39 RBIs, and 38 stolen bases in his inaugural season. His 38 stolen bases total ranked eighth in the National League. He hit his first major league grand slam on August 23.
In the NL Championship Series against St. Louis, he hit .222 (4-for-18) with two doubles and a triple.
However when Duncan’s next three seasons, 1986-1988, fell below expectations he played mostly in the minors, and was traded to the Cincinnati Reds in mid-1989. His bat got hot in 1990, and so did the Reds. Mariano Duncan hit a very fine .306 and led the NL with 10 triples as Cincinnati made it to the World Series and then swept Oakland in 4 games. In the NLCS vs Pittsburgh, Duncan batted .300, with 6 hits in 20 at bats, and powered a 3-run homer in Game 3. In the Series he was 2-for-14 with one RBI. In 1991 he played second base.
Prior to 1992 Mariano Duncan inked a free agent contract with the Phillies.
As a youngster Mariano Duncan played baseball with Juan Samuel and Tony Fernandez in
San Pedro de Macoris, Dominican Republic.