Mostil debuted for Chicago in 1918 but was sent back down to the minors. He would not return until 1921, and he showed off good defense and a capable batting eye. He batted over .300 in 1921, 1922, 1924, and 1926, but it was in 1925 (where he batted .299) where he would have his best season. The Centerfielder would lead the American League in Runs Scored (135), Stolen Bases (43) and Walks (43). For his efforts, Mostil finished second in MVP voting.
Everything went downhill from there as health problems and injuries reduced him to 12 Games, and while he came back for a full season in 1928, injuries took him out early in 1929, and he would never return to the Majors again.Overall, Mostil would have 1,054 Hits, 176 Stolen Bases with a .301 Batting Average.