Prior to his political career, MacMillan worked as an insurance agent between 1969 and 1972. He was also the owner of a hotel from 1972 to 1983 and the co-owner of Nord MacMillan Inc. in 2003 which operates the same hotel business. He was also the Chair of the Economic Development Committee for the region and also a city councillor for the former city of Buckingham in the 1980s.
In addition, of being involved in the economic development of the Buckingham and La Lievre region, MacMillan was also involved in the community as he is a member of the Knights of Columbus Buckingham Division since 1968 and the member of the Lions Club since 1973. He also founded a Junior A hockey club, les Castors de Buckingham, in 1973.
MacMillan entered provincial politics as he was elected MNA for Papineau in a by-election in 1989 although he was a member of the Liberal Party since 1980. Since then he was re-elected in 1994, 1998, 2003 and 2007. After being briefly the president of the government's caucus, he was named the government's chief whip in 2005. He never held a Minister's portfolio during his 18-year career, nor was he a Parliamentary Secretary for any Ministers.
MacMillan is married and has two children.