He began his career as a starting pitcher, pitching in the Reds' organization for four seasons, never rising above Class A ball.

The Mets moved him steadily up the minor league ladder over the next three seasons, and Contreras reached Triple-A with the Tidewater Tides in 1975.

He returned to Iowa in 1980, this time posting better numbers (9-7, 4.19 ERA in 20 games, all as a starter), and got his first, and what would prove to be his only, shot at the major leagues.

Contreras returned to the minors for the White Sox, pitching sparingly over the next two seasons for their new Triple-A affiliate, the Edmonton Trappers.

He was the pitching coach for the New York Yankees in 1995, the Seattle Mariners in 1997–98, and then with the team that gave him his shot at the majors, the White Sox, from 1998 until 2002.