MacMillan, 25, was a fourth-round pick (113th overall) of the Montreal Canadiens in the 2010 NHL Entry Draft. A native of Penticton, British Columbia, he has played the last two seasons with the American Hockey League's St. John's IceCaps, where he tallied 30 points (12g-18a) in 120 games. He also had five points (2g-3a) in six games with the Brampton Beast during the 2015-16 campaign.
Prior to turning pro, MacMillan played four years at the University of North Dakota and served as an alternate captain during his senior season. He was named Defensive Forward of the Year in the National Collegiate Hockey Conference in 2014-15 and was leading the conference with a career-high 16 goals when he suffered an injury in February. MacMillan finished his college career with 99 points (46g-53a) in 151 games.