Born in Bradford, Mitchell was educated at Woodbottom Council School, Bingley Grammar School, the University of Manchester (BA; MA) where he read History, and Nuffield College, Oxford, graduating with a Doctor of Philosophy degree on "The Whigs in opposition, 1815–1830" in 1963.
From 1959–63, he lectured in history at the University of Otago in Dunedin, New Zealand. While lecturing in sociology, from 1963–67, at the University of Canterbury, Mitchell wrote a popular book about New Zealand, The Half Gallon Quarter Acre Pavlova Paradise (1972). The book title became a phrase in the New Zealand English lexicon. In the 1960s and 70s, New Zealand remained a milder version of the socialist laboratory it had been since 1935. In the 80s and 90s, the same socialist Labour party's government transformed it into an open market economy. These drastic changes provided ample subject matter for social analysis and, 30 years later, Mitchell wrote Pavlova Paradise Revisited (2002) as well as a video series accessible on NZ on Screen, after another New Zealand expedition. From 1967–69, Mitchell was an Official Fellow at Nuffield College.
Mitchell was a founding member of the University of Canterbury Political Science Department in 1963, supporting it breaking away from former conjoint with the History. In 2015, he returned to the University of Canterbury as a Canterbury Visiting Fellow. The Canterbury Visiting Fellowship is generously paid for by the University of Canterbury and administered by the Erskine Programme. Austin is lecturing on "Britain and New Zealand - The Great Unravelling' looking at the evolution of recent British politics, drawing analogies in each section with parallel developments and implications for New Zealand to examine all worldwide trends in the evolution of liberal English speaking democracies.
He was a journalist at ITV company Yorkshire Television from 1969 to 1977, presenting their regional news programme Calendar, although he spent a short period at the BBC in 1972. During his period at Yorkshire, Mitchell chaired a tense live studio discussion involving Brian Clough and Don Revie, immediately following Clough's sacking by Leeds United in 1974.
He was elected to Parliament at a by-election in 1977, following the death of the previous MP, the Foreign Secretary Tony Crosland. At the time Mitchell identified himself as a Gaitskellite.
Mitchell supported the introduction of television cameras to the House of Commons, raising it for discussion in 1983. The move opened the proceedings of the House to the wider public, who previously had only been able to follow via newspapers and, from 1978, radio.
In October 2002, he temporarily changed his name to Austin Haddock as haddock is a staple catch for his constituents that was suffering a decline and it was his wish to promote it.
He was chair of the Parliamentary All-Party Photography Group and campaigned for the recognition of photographers' rights after an over-zealous police officer deleted photographs, without his permission from his camera's memory card at the 2005 Labour Conference in Brighton.
In 2007, Mitchell wrote a front-page article for The Independent newspaper in which he criticised the treatment of a family of asylum-seekers in his constituency. This article quoted him as saying that certain correspondents on the subject to the website of the local newspaper, the Grimsby Telegraph, were "lumpen lunatics." The Grimsby Telegraph covered the response in which it stood by the MP but also reported that a number of readers had called for his resignation.
He is a member of the Socialist Campaign Group – although this affiliation did not prevent him from nominating Gordon Brown (rather than John McDonnell) for the 2007 Labour Party leadership election. As a supporter of the Better Off Out campaign, Mitchell is a Eurosceptic and he opposes the Common Fisheries Policy. Mitchell is also a keen supporter of the Additional Member System, (the electoral system used in elections to the Scottish Parliament and Welsh Assembly), and called a Private Members' Debate on this issue on 1 December 2009.
During 2010, Mitchell participated in Tower Block of Commons, a Channel 4 documentary where MPs live in tower blocks and in with ordinary residents in deprived areas. Mitchell, who insisted on living in his own flat with his wife instead of living with the local residents, was criticised for his apparent lack of engagement in comparison to his Liberal Democrat and Conservative counterparts. He claimed the production company misled him. Mitchell is the President of the Debating Group.
In April 2014, Mitchell announced that he would not be standing in the next general election.
As part of an independent audit conducted after the United Kingdom Parliamentary expenses scandal of 2009, in which expense claims between 2004 and 2008 for second homes were examined, Mitchell was discovered to have wrongly claimed £10,549 for mortgage repayments. He explained that this was as a result of an oversight in 2006; in January 2010, he issued an apology and repaid the funds.
On 29 October 2012, Mitchell directed a tweet at former Conservative MP Louise Mensch, saying "A good wife doesn’t disagree with her master in public and a good little girl doesn’t lie about why she quit politics." He also referred to Mensch as "Menschkin." The comments were widely condemned as being sexist, with Mensch demanding an apology from both Mitchell and Ed Miliband. Mitchell claimed he was being "ironic".
Austin lives with his second wife, the journalist and author Linda McDougall, whom he married in 1976 in Rochdale and with whom he has a son and daughter. He was previously married to Patricia, with whom he had 2 daughters.
Mitchell has taken photographs for and co-authored the book Parliament in Pictures: Inside the House of Commons and the House of Lords (ISBN 9780500019597).
In July 2013, Mitchell underwent heart surgery at King's College Hospital, London, to repair a leaking valve.