In the 1970 general election, Mullin, aged 22, stood unsuccessfully against Liberal Leader Jeremy Thorpe in North Devon. By 1980 he was an executive member of the Labour Co-ordinating Committee. As such he was an active supporter of Tony Benn when, in 1981, disregarding an appeal from party leader Michael Foot to abstain from inflaming the party's divisions, Benn stood against the incumbent Deputy Leader of the Labour Party, Denis Healey. In addition Mullin edited two collections of Benn's speeches and writings Arguments for Socialism (1979) and Arguments for Democracy (1981).
Before being elected as an MP, he was a journalist working for the Granada current affairs programme World in Action and was pivotal in securing the release of the Birmingham Six, a long-standing miscarriage of justice. He was also editor of the Tribune newspaper (1982-84). His novel A Very British Coup was published in 1982. It describes the destabilisation (and ultimate replacement) of a left wing British government by the forces of the Establishment. The novel was adapted for television by Alan Plater, with substantial alterations to the plot, and screened in 1988.
Mullin is a graduate of the University of Hull.
Mullin was first elected MP in 1987, and was returned at every subsequent election until 2005. He did not seek re-election in 2010.
His constituency has been the first to declare in every general election since 1992 (1992, 1997, 2001, and 2005 2010). Mullin jokes about being the UK's sole MP for a few minutes and muses about forming a government.
Chris Mullin first visited Cambodia in 1973, and again in 1980; in 1989 and 1990, he was outspoken on the British Government's record in Cambodia, being a leading voice in some of the first protracted debates on the UK's provision of military support to the Khmer Rouge, and attributing increasing public interest in the issue to the documentary films of John Pilger.
He was a member of the Socialist Campaign Group, Secretary of the All-Party Parliamentary Group for Vietnam and Chair of the All-Party Parliamentary Group for Cambodia, Member of the Home Affairs Select Committee (1992—97), Chairman of the Home Affairs Select Committee (1997—99).
Despite his criticism of the government, he replaced Alan Meale as Parliamentary Under-Secretary of State at the DETR in July 1999 before taking over from George Foulkes as Parliamentary Under-Secretary, Department for International Development in 2001.
He returned to government in June 2003, as a Parliamentary Under-Secretary at the Foreign Office, but after the 2005 election again returned to the backbenches. Before the Labour victory of 1997, Mullin had attained a reputation for campaigning on behalf of victims of injustice and opposition to the curtailing of civil rights. His campaigning stance had to change while a minister because of the collective responsibility of government. His vote against the government's proposal for 90-days detention without trial for terrorist suspects, as one of 49 Labour rebels, seemed to indicate a re-emergence of his civil libertarian instincts. He criticised the Labour government's commitment to its expressed policy on Africa.
During the UK Parliamentary expenses scandal Mullin, one of the lowest claimers, provided some comic relief when it was revealed that the television at his second home is an ancient black-and-white model with a £45 TV licence.
On 10 May 2008, the Sunderland Echo site reported that Mullin had decided to stand down at the 2010 general election.