MacMillan spent four seasons in the Ontario Hockey League with three different teams and turned pro in 2003. He played in the ECHL and split the 2003-04 season with the Peoria Rivermen, Cincinnati Cyclones and the Florence Pride. After spending 2004-05 in the United Hockey League with the Port Huron Beacons, MacMillan returned to the ECHL with the Reading Royals. He also had short spells in the American Hockey League with the Hershey Bears and the Albany River Rats. In 2008, MacMillian moved to the United Kingdom, signing with the Belfast Giants scoring 36 points in 47 games. 