On April 17, 1954, he became the first African American to play for the Cincinnati Reds' franchise, known during the mid-1950s as the Redlegs. Pinch-hitting for pitcher Corky Valentine against Lew Burdette, Harmon flied out in the seventh inning of a 5-1 loss to the Milwaukee Braves at County Stadium. In that game he came to bat after another rookie, Nino Escalera, an Afro-Latin American from Puerto Rico who pinch hit immediately before Harmon to become the first black player to appear for the Cincinnati franchise.
Harmon got his first hit on April 25, 1954. Starting and leading off for the Reds at home in Crosley Field in a 3-2 win over the Chicago Cubs, Harmon singled in the first inning off Howie Pollet. He later doubled and scored on an error, and drew one walk. His final game was the site of his first, Milwaukee's County Stadium on September 15, 1957 where, appearing as a pinch runner for the Philadelphia Phillies, he scored his final run on a double play.