Raised in Cleburne, Texas, McWilliams long tenure with the Longhorn football program started as a player from 1961 to 1963, when he helped the team compile a record of 30–2–1. During that time, the squad won a national championship, three Southwest Conference championships, and finished the year ranked among the nation's top four each year. He served as a tri-captain during the 1963 season.