MacMillan played seven NHL seasons, his first two being with the Toronto Maple Leafs from 1970 to 1972, before one season with the Atlanta Flames and four as a New York Islander. He'd rack up 151 points in 446 NHL games before retiring as a player in 1978.
He'd get hired by the Islanders as an assistant coach in 1979 before becoming head coach of the Colorado Rockies the following year before their relocation to New Jersey as the Devils, where he'd spend two more seasons as head coach.