Glen's major film roles include starring as a villain in two Hollywood franchises adapted from video games; first as Manfred Powell in Lara Croft: Tomb Raider, and later as Dr. Sam Issacs in Resident Evil: Apocalypse, and Resident Evil: Extinction. More recently he also appeared in Kick-Ass 2.
He has had several guest and recurring roles in various British TV series, such as Spooks and Downton Abbey. He also portrayed the eponymous Jack Taylor in various TV specials, as well as playing Ser Jorah Mormont in HBO's Game Of Thrones.
As well as appearing on screen, Glen also regularly acts on stage, and has received Olivier award nominations for his performances in Martin Guerre, The Blue Room and The Crucible.