Known for his character work,[2] he has played supporting roles in films such as Highlander (1986), Braveheart (1995), Trainspotting (1996), Troy (2004), The Chronicles of Narnia: The Lion, the Witch and the Wardrobe (2005), Ben-Hur (2016), and Wonder Woman (2017).

[5] Cosmo is best known for his film roles as Angus MacLeod in Highlander, Campbell in Braveheart and as Father Christmas in the adaptation of The Chronicles of Narnia: The Lion, the Witch and the Wardrobe.

[8] He also appeared in UFO as SHADO operative Lieutenant Anderson, in Take the High Road as Alex Geddes from 1982 until 1983 and in 1984 played Jock McLeish in the Minder episode Senior Citizen Caine.

[10] He was involved in the filming of the thriller Breakdown in which he appeared alongside Craig Fairbrass, Bruce Payne, Emmett Scanlan, Olivia Grant and Tamer Hassan.

[12] He played a Russian intelligence officer named Luka Gocharov in season 3 of the Amazon show Jack Ryan.