Recognized as a solid, bodychecking forward, Billy MacMillan left his home in Prince Edward Island to play junior hockey at Toronto's St.Michael's College, followed by a year at Neil McNeil after St.Mike's pulled out of Junior A competition in 1962.
MacMillan went to the Memorial Cup three times in his three full years of junior hockey competition. After graduating from the OHL, he played four years with the Canadian National Team and earned a degree in Physical Education at the University of Manitoba at the same time.
In 1970-71, MacMillan finally got his chance, making the Toronto Maple Leafs and scoring 22 goals in his rookie season, what would be the highest output of his NHL career. After two seasons with the Leafs, MacMillan was claimed by the Atlanta Flames in the Expansion Draft. In 1973, he was traded to the New York Islanders, where he played for four seasons before finishing his playing career. But his career in hockey was far from over.
MacMillan coached in the Islanders' organization, winning the AHL's Adams Cup with the Fort Worth Texans, and was named Coach of the Year. With that success in his first season coaching, MacMillan was promoted to an assistant coaching position under Al Arbour in 1979-80. The Islanders won the Stanley Cup that season, yet at the conclusion of the playoffs, he was lured away to join the Colorado Rockies as their head coach, succeeding the popular and colourful Don Cherry.
MacMillan lasted just one year, but in 1982-83, he was hired as coach and general manager of the New Jersey Devils, where he stayed for just over a year before being released.