[5] For a brief period, Mullan was a member of a street gang while at secondary school,[5][6] and worked as a bouncer in a number of south-side pubs.

He had roles in films alongside actors such as Robert Carlyle in Riff-Raff (1991),[10] Shallow Grave (1994),[10] with Mel Gibson in Braveheart (1995),[10] and with Ewan McGregor in Danny Boyle's Trainspotting (1996).

[12] He won the World Dramatic Special Jury Prize for Breakout Performances at 2011 Sundance Film Festival for his work on Paddy Considine's Tyrannosaur (2011).

[13] Mullan has appeared as supporting or guest actor in numerous cult movies, including Session 9 (2002),[10] Young Adam (2003),[10] Children of Men (2006),[10] the final two Harry Potter films (2010–2011),[10] and War Horse (2011).

[15] A self-described Marxist,[5] Mullan continues to support socialist causes and was a leading figure in the left-wing theatre movement that blossomed in Scotland during the Margaret Thatcher and John Major Conservative governments in the 1980s and early to mid-1990s.

Mullan took part in a 2006 occupation of the Glasgow offices of the UK Immigration Service, protesting against the UKIS's "dawn raid" tactics when deporting failed asylum seekers.

[17] In January 2009, Mullan joined other actors in protesting against the BBC's refusal to screen a Disasters Emergency Committee appeal for Gaza.

They told BBC director general Mark Thompson: "Like millions of others, we are absolutely appalled at the decision to refuse to broadcast the appeal.

"[18] Mullan has agreed to appear in an adaptation of Iain Banks's novel Stonemouth after the BBC aired a DEC appeal for Gaza in late 2014.