He is best known for his performance as numerous clones in the Space and BBC America science fiction television series Orphan Black (2014–2017), for which he won a Canadian Screen Award in 2016.

In secondary school at Kingston Collegiate and Vocational Institute, he spent his Ontario Academic Credit year studying theatre.

[2] Millen signed with a talent agency in his fourth year of university, and one of his first roles was in a 2008 episode of the television series ReGenesis.

The Veil opened in Halifax before relocating to Iran in January 2009, where the play was featured in Tehran's Fajr International Theater Festival.

[2] From 2010 to 2013, Millen appeared in a variety of small roles in the television series Nikita, Rookie Blue and State of Syn, as well as the horror films Monster Brawl and Exit Humanity.