In the summer after his senior year, he played as a catcher for the Buick Colts of the Michigan American Legion, contributing to the team winning the city and state championships.

Unbeknownst to the Cowboys, he had signed a professional baseball contract with the Baltimore Orioles of MLB one day earlier on November 28.

After the trade of Frank Robinson to the Los Angeles Dodgers in early December 1971, Rettenmund began the 1972 season as the Orioles' starting right fielder.

By 1973, he was out of the starting lineup due to injuries, prolonged batting slumps and the emergence of Al Bumbry and Rich Coggins.

[3] Rettenmund, along with Junior Kennedy and Bill Wood, was sent to the Cincinnati Reds for Ross Grimsley and Wally Williams on December 4, 1973.

After his major league playing career, Rettenmund served as hitting coach for the Texas Rangers (1983–85), the Athletics (1989–90), the Padres (1991–99), the Atlanta Braves (2000–01), and the Detroit Tigers (2002).

After three years out of the majors, Rettenmund returned as hitting coach of the Padres in June 2006, replacing Dave Magadan.