For several years thereafter Killilea served as the league's legal counsel. The American League started play as a major league in 1901. Killilea owned the league's Milwaukee Brewers franchise that first season, but during the offseason he sold the club, which was moved to St. Louis and renamed the St. Louis Browns before the 1902 season. Killilea also owned the Boston Red Sox from 1903 until 1904. In 1928 he acquired the Milwaukee Brewers, in this incarnation a minor-league American Association club.