Keir Starmer was born in Southwark on 2 September 1962. Starmer was the second of four children of Josephine (née Baker), a nurse, and Rod Starmer, a toolmaker. He was named after the first Labour MP, Keir Hardie. He passed the 11-plus examination and gained entry to Reigate Grammar School, then a voluntary aided school. He studied law at the University of Leeds and graduated with a first class Bachelor of Laws (LLB) degree in 1985. He then undertook postgraduate studies at St Edmund Hall, Oxford, and graduated from the University of Oxford with a Bachelor of Civil Law (BCL) degree in 1986.
Starmer became a barrister in 1987. He advised Helen Steel and David Morris in the McLibel case, which went to court in 1997. In an interview, he described the case as "very much a David and Goliath", and said that "there's an extremely good legal team acting for McDonalds at great expense and Dave and Helen have had to act for themselves with me as a sort of free back up whenever possible." He was also interviewed for McLibel, the documentary about the case directed by Franny Armstrong and Ken Loach. He was appointed Queen's Counsel in 2002, and was joint head of his chambers, Doughty Street Chambers.
He was a human rights adviser to the Northern Ireland Policing Board and the Association of Chief Police Officers. He is a member of the Foreign Secretary's Death Penalty Advisory Panel. In 2007, he was named "QC of the Year".
On 25 July 2008, the Attorney General, Patricia Scotland, named Starmer as the next head of the CPS, to take over from Ken Macdonald on 1 November 2008. Macdonald, himself a former defence lawyer, welcomed the appointment. While in office, he was viewed as a Labour supporter.
On 22 July 2010, Starmer announced the decision not to prosecute the police officer Simon Harwood in relation to the death of Ian Tomlinson; this led to accusations by Tomlinson's family of a police cover-up. After an inquest found that Mr Tomlinson had been unlawfully killed, Starmer announced that Harwood would be prosecuted for manslaughter. In July 2012 he was acquitted, but he was dismissed from the police in September 2012.
On 3 February 2012, Starmer announced that the energy secretary, Chris Huhne and his former wife, Vicky Pryce would be prosecuted for perverting the course of justice. Huhne became the first Cabinet minister in British history to be compelled to resign as a result of criminal proceedings. Starmer had previously said in relation to the case that "[w]here there is sufficient evidence we do not shy away from prosecuting politicians".
In the summer of 2012, Nick Cohen, a journalist, published allegations that Starmer was personally responsible for the continued prosecution of Paul Chambers, an airline passenger who, frustrated at airport delays, had posted a joke about Doncaster Sheffield Airport on Twitter. In the case known as the "Twitter Joke Trial" Chambers had been convicted of sending a message "of a menacing character". However, the CPS said the decision was out of Starmer's hands as it was a Crown Court decision.
He left office on 1 November 2013 and was replaced by Alison Saunders.
In December 2013, the Labour Party announced that Starmer would lead an enquiry into changing the law to give further protection to victims in cases of rape and child abuse. On 28 December, Starmer said to BBC News, "Well, I'm back in private practice; I'm rather enjoying having some free time, and I'm considering a number of options".
Starmer was selected on 13 December 2014 as the Labour Party's prospective parliamentary candidate for the constituency of Holborn and St Pancras, following the decision of the sitting MP Frank Dobson to stand down. Starmer was elected at the 2015 general election with a majority of 17,048.
He was urged by activists to stand to be leader of the Labour Party in the 2015 leadership election following the resignation of Ed Miliband, but he ruled out doing so, citing his lack of political experience. During the campaign, Keir Starmer supported Andy Burnham.
Jeremy Corbyn appointed Starmer as a shadow Home Office minister on 18 September 2015. On 27 June 2016 he resigned as shadow minister in protest over the leadership of the Labour Party, and supported Owen Smith in the September 2016 Labour Party leadership election.
On 6 October 2016, Starmer was appointed by Labour leader Jeremy Corbyn as shadow Brexit secretary, replacing Emily Thornberry in this role. Starmer resigned from a consultancy position with the law firm specialising in human rights (Mishcon de Reya LLP) that acted for Gina Miller in bringing legal proceedings against the Brexit secretary. Starmer has used his position as shadow Brexit secretary to question the government's "destination" for Britain outside the European Union, as well as calling for the government's Brexit plan to be released. On 6 December 2016, Prime Minister Theresa May confirmed the government would do this, in what was portrayed as a victory for Starmer.
Starmer has questioned whether the victory for "leave" in the 2016 European Union membership referendum was a mandate for a so-called "hard Brexit", which would see the UK leave the European Single Market and not just the Political union itself. He said that Theresa May together with the government would be subject to a race against time to pass a large number of new laws, or risk an "unsustainable legal vacuum", if Britain left the EU without a deal. On 25 September 2018, he announced to the Labour Party Conference in Liverpool that "campaigning [by the party] for a public vote must be an option".