Cooper’s career was delayed some as he was drafted for WWI. About the only thing known now about his Army career was that he served in France during the war. His SABR biography says that Cooper was the ace of the Detroit Stars from 1923 to 1927. J.L. Wilkinson, the owner of the Kansas City Monarchs traded a bunch of players to get the 30-year-old lefty.
Cooper had two solid seasons with the Monarchs, helping them to second place the first season and first place the second season. The Monarchs also had two young pitchers help them win the pennant the second season. It seems with enough pitching Wilkinson let Cooper go. Cooper then signed back with Detroit. Cooper had an even better season with Detroit and helped them to the second half championship.
After that season Cooper pitched for independent teams including for the Monarchs again starting in 1932. He also started to manage them in 1935. In 1937 the Monarchs joined the Negro American League. By that time Cooper was 40 years old and made only occasional appearances.