Millar, 23, finished the 2023-24 campaign with Ontario after signing a professional tryout agreement (PTO) with the team to begin his professional career on Apr. 9. The blueliner made his pro debut later in the month in Colorado on Apr. 21 during the final game of the regular season. Before joining the Reign, Millar completed his senior season in the NCAA at Colorado College. He scored 12 points (4-8=12) in 37 games with the Tigers while serving as an alternate captain and finished with 60 shots on net and one game-winning goal in helping CC to a 21-13-3 record.
He appeared in 134 career NCAA games from 2020-24 at Colorado College and finished with 31 points on eight goals and 23 assists. Millar was in the lineup for all his school's contests in every one of his four collegiate campaigns.