Mitchell was born in Cambridge, Massachusetts. In 1917, he joined the Chicago Cubs as team president, and was later hired as manager. In his second year at the helm, he won the 1918National League pennant, losing to the Red Sox in the 1918 World Series. However, in the middle of the 1919 season, he was relieved of his president duties and one year later, he was out of a job. The Boston Braves hired him as manager for the 1921 season, but his success in Chicago did not follow him to his hometown Braves, where he lost 100 games twice. After he was fired by the Braves, he returned to Harvard University where he had previously coached baseball in 1916. He remained at Harvard for thirty years until his retirement. Mitchell was best known for his excellence in coaching.