Born in Warrington, Postlethwaite trained as a teacher and taught drama at Manchester's Loreto College before studying at Bristol Old Vic Theatre School. His early acting career at the Liverpool Everyman saw him work with actors such as Bill Nighy, Jonathan Pryce, Antony Sher, Julie Walters and Alan Bleasdale.
Having joined the Royal Shakespeare Company in the mid 1980s, Postlethwaite's first major film role was the 1988 British feature Distant Voices, Still Lives. He broke into Hollywood in the early 90s, appearing alongside Mel Gibson in Hamlet as well as roles in Alien 3 and The Last of the Mohicans.
Amongst his notable stage roles was his performance in Justin Butcher's one-man play Scaramouche Jones. Directed by Rupert Goold in 2002, the play, which tells the life story of a clown on the eve of both the new Millenium and his hundredth birthday, toured the UK and extensively overseas.
His films roles included the 1996 film Brassed Off, a favourite of the former deputy prime minister John Prescott, in which Postlethwaite played the leader of a Yorkshire colliery band in a community devastated by mine closures. Prescott went on to say the film inspired New Labour's coalfield regeneration programme.
In recent years Postlethwaite's political activism became a more prominent part of his public persona, starring in the 2009 climate change film The Age of Stupid, marching against the Iraq war and supporting the Make Poverty History campaign.
Having been awarded the OBE in the 2004 New Year's Honours List, he promised at the 2009 premiere for The Age of Stupid that would return this medal if the government went ahead with plans for a new coal-fired power station at Kingsnorth, Kent. He arrived at the film's premiere on a bicycle.
Postlethwaite's performance in Liverpool Everyman's production of King Lear was one of the highlights of Liverpool's 2008 year as the European Capital of Culture. The production also transferred to London, playing the Young Vic in early 2009 and fulfilled Postlethwaite's long held wish to play the role.
He is survived by his wife, Jacqueline Morrish and their two children.