His first full season was with the Sault Thunderbirds of the Eastern Professional Hockey League in 1960–61. Stapleton had signed with the Chicago Black Hawks, but was claimed by the Boston Bruins in the intra-league draft in June 1961 and began his National Hockey League career with the Bruins in the 1961–62 season. The next year, he split his time between Bruins and their EPHL affiliate, the Kingston Frontenacs. Stapleton spent the next two years in the minor leagues, playing with the Portland Buckaroos of the Western Hockey League. He received the Hal Laycoe Cup as the WHL's top defenceman for the 1964–65 season.