Cox was born Helen Joanne Leadbeater on 22 June 1974 in Batley, West Yorkshire, England, to Jean, a school secretary, and Gordon Leadbeater, a toothpaste and hairspray factory worker.

Four hours after the incident, West Yorkshire Police announced that Cox had died of her injuries shortly after being admitted to Leeds General Infirmary. She was the first sitting MP to be killed since the Conservative Party’s Ian Gow, who was killed by a Provisional IRA car bomb in July 1990, and the first MP to be seriously assaulted since Stephen Timms, who was stabbed by Roshonara Choudhry in an attempted assassination in May 2010. A memorial service was held at St Peter’s Church in Birstall the following day.

Raised in Heckmondwike, she was educated at Heckmondwike Grammar School, a state grammar school, where she was head girl. During summers, she worked packing toothpaste. Cox studied at Pembroke College, Cambridge, initially studying Archaeology and Anthropology before switching to Social and Political Science, graduating in 1995. She later studied at the London School of Economics.

Following her graduation from Pembroke College, Cox worked as an adviser to Labour MP Joan Walley, before moving to Brussels to spend two years as an assistant to Glenys Kinnock, wife of former Labour leader Neil Kinnock, who was then a Member of the European Parliament. From 2001 to 2009, Cox worked for the aid groups Oxfam and Oxfam International, first in Brussels as the leader of the group’s trade reform campaign, then as head of policy and advocacy at Oxfam GB in 2005, and head of Oxfam International’s humanitarian campaigns in New York City in 2007. While there, she helped to publish For a Safer Tomorrow, a book authored by Ed Cairns which examines the changing nature of the world’s humanitarian policies. Her work for Oxfam in which she met disadvantaged groups in Darfur and Afghanistan influenced her political thinking.

The assassination received worldwide attention with tributes and memorials for Cox being made with condemnation of Mair. A personal friend, Canadian MP Nathan Cullen paid tribute to Cox in the House of Commons of Canada. Canadian Prime Minister Justin Trudeau, former US Secretary of State Hillary Clinton, the then US Secretary of State John Kerry and former US Representative Gabrielle Giffords, who was wounded in an assassination attempt in 2011, were among international politicians who sent messages of condemnation and sympathy in the aftermath of her killing. Cox’s husband issued a statement urging people to “fight against the hatred that killed her”.

Cox was nominated by the Labour Party to contest the Batley and Spen seat being vacated by Mike Wood in the 2015 general election. She was selected as its candidate from an all-women shortlist. The Batley and Spen seat was a Conservative marginal between 1983 and 1997, but was considered to be a safe seat for Labour, and Cox won the seat with 43.2% of the vote, increasing Labour’s majority to 6,051. Cox made her maiden speech in the House of Commons on 3 June 2015, using it to celebrate her constituency’s ethnic diversity, while highlighting the economic challenges facing the community and urging the government to rethink its approach to economic regeneration. She was one of 36 Labour MPs who nominated Jeremy Corbyn as a candidate in the Labour leadership election of 2015, but said she had done so to get him on the list and encourage a broad debate. In the election she voted for Liz Kendall, and announced after the local elections on 6 May 2016 that she and fellow MP Neil Coyle regretted nominating Corbyn.

Cox campaigned for a solution to the Syrian civil war. In October 2015, she co-authored an article in The Observer with Conservative MP Andrew Mitchell, arguing that British military forces could help achieve an ethical solution to the conflict, including the creation of civilian safe havens in Syria. During that month Cox launched the All Party Parliamentary Friends of Syria group, becoming its chair. In the Commons vote in December to approve UK military intervention against ISIL in Syria, Cox abstained because she believed in a more comprehensive strategy that would also include combatting President Bashar al-Assad and his “indiscriminate barrel bombs”. She wrote:

Andrew Grice of The Independent felt that she “argued forcefully that the UK Government should be doing more both to help the victims and use its influence abroad to bring an end to the Syrian conflict”. In February 2016, Cox wrote to the Nobel Committee praising the work of the Syrian Civil Defense, a civilian voluntary emergency rescue organisation known as the White Helmets, and nominating them for the Nobel Peace Prize: “In the most dangerous place on earth these unarmed volunteers risk their lives to help anyone in need regardless of religion or politics”. The nomination was accepted by the committee, and garnered the support of twenty of her fellow MPs and several celebrities, including George Clooney, Daniel Craig, Chris Martin and Michael Palin. The nomination was supported by members of Canada’s New Democratic Party, who urged Stéphane Dion, the country’s Foreign Affairs Minister, to give his backing on behalf of Canada.

Cox, a supporter of the Labour Friends of Palestine & the Middle East, called for the lifting of the blockade of the Gaza Strip. She opposed efforts by the government to curtail the Boycott, Divestment and Sanctions movement, and said “I believe that this is a gross attack on democratic freedoms. Not only is it right to boycott unethical companies but it is our right to do so.” Cox was working with Conservative MP Tom Tugendhat on a report to be published following the release of the Chilcot Report into the 2003 invasion of Iraq. After her death, Tugendhat wrote in The Times, “Our starting point was that while Britain must learn the painful lessons of Iraq, we must not let the pendulum swing towards knee-jerk isolationism, ideological pacifism and doctrinal anti-interventionism”. With the charity Tell MAMA she worked on The Geography of Anti-Muslim Hatred, investigating cases of Islamophobia; the report was dedicated to her at its launch on 29 June 2016. Two parliamentary questions concerning the Yemeni Conflict, tabled by Cox to the Foreign and Commonwealth Office on 14 June, were answered by Under-Secretary of State for Foreign Affairs, Tobias Ellwood after her death. On 1 July, The Guardian reported that each answer was accompanied by a government note stating “This question was tabled before the sad death of the honourable lady but the subject remains important and the government’s response ought to be placed on the public record.”

Cox was married to Brendan Cox, an adviser on international development to Gordon Brown during Brown’s premiership, whom she met while she was working for Oxfam, from June 2009 until her death in June 2016. They had two children. The Cox family divided their time between their constituency home and a houseboat, a converted Dutch barge, on the Thames, moored near Tower Bridge in London. A secular humanist, Jo was a supporter of the British Humanist Association.

On 23 November 2016, Mair was found guilty of all charges – the murder of Cox, stabbing Bernard Kenny (a charge of grievous bodily harm with intent), possession of a firearm with intent to commit an indictable offence and possession of an offensive weapon, namely the dagger. The trial judge said that the crime was so severe that 53-year-old Mair should be given a whole-life tariff—never be released from prison, except at the discretion of the Home Secretary.

Among those who paid tribute to Cox were Labour Party leader Jeremy Corbyn, who described her as someone who was “dedicated to getting us to live up to our promises to support the developing world and strengthen human rights”, while Prime Minister David Cameron said she was “a star for her constituents, a star in parliament, and right across the house”. US President Barack Obama telephoned Cox’s husband to offer his condolences, noting that “the world is a better place because of her selfless service to others”. Parliament was recalled on 20 June 2016 for fellow MPs to pay tribute to Cox.

A by-election in Batley and Spen was held on 20 October 2016. Labour candidate Tracy Brabin, an actress whose credits include a role in Coronation Street in the mid 1990s, won the by-election with 86 percent of the vote. The Conservative Party, Liberal Democrats, Green Party, and UKIP did not contest the election as a mark of respect. Far-right candidate and former British National Party member Jack Buckby caused widespread condemnation by standing in the by-election, with Cox’s former Labour colleague MP Jack Dromey describing Liberty GB’s bid as “obscene, outrageous and contemptible”.

In December 2016, a group of politicians came together to record a cover of the Rolling Stones “You Can’t Always Get What You Want” in honour of Cox. Politicians from the Labour Party, the Conservatives, and the SNP joined with members of the Parliament Choir, the Royal Opera House Thurrock Community Chorus, KT Tunstall, Steve Harley, Ricky Wilson, David Gray and other musicians. All profits from sales of the song went to the Jo Cox Foundation. The single raised over £35,000 for the Jo Cox Foundation and was in the iTunes top 10 after its release but was placed 136 in the Christmas chart.

One year after her murder, three individuals who came to her aid were honoured in the 2017 Queen’s Birthday Honours. Bernard Kenny, a passerby who tried to stop Mair during the attack and was himself stabbed in the stomach, was awarded the George Medal, which is given to civilians who exhibit great bravery. PC Craig Nicholls and PC Jonathan Wright of the West Yorkshire Police, who apprehended and arrested her attacker after he had fled the scene, were awarded the Queen’s Gallantry Medal.

In May 2017, a memorial, designed by Cox’s children, was unveiled in the House of Commons. The unveiling took place at the first “Great Get Together” event that the Jo Cox Foundation held and was in the form of a family day at Parliament. In June 2017, Cox’s husband Brendan published Jo Cox: More In Common, a book that talks about the impact of his wife’s death on their family. Also in June 2017, and to mark the first anniversary of Cox’s death, her family and friends urged people to take part in a weekend of events to celebrate her life and held under the banner of “The Great Get Together”; events included picnics, street parties and concerts.

A street, formerly Rue Pierre–Étienne Flandin after Pierre-Étienne Flandin, in Avallon, a town in the Yonne département of France, was renamed Rue Jo Cox in May 2017. In Brussels, a square beside the Ancienne Belgique concert hall was renamed to Jo Coxplein in September 2018.

A work of contemporary dance theatre inspired by Cox’s political and social beliefs, entitled “More in Common”, was created by Youth Music Theatre UK in August 2017 and presented at the Square Chapel, Halifax.

Her alma mater, Pembroke College, announced a Jo Cox Studentship in Refugee and Migration Studies, which was first awarded in 2017 after extensive fundraising by members of the college.

On 24 June 2017, a coat of arms, designed with the input of Cox’s children, was unveiled by her family at the House of Commons, where MPs killed in office are honoured with heraldic shields. The elements of the arms included four roses, to symbolise the members of Cox’s family (two white roses, for Yorkshire, and two red, for Labour); and the tinctures green, purple, and white, which were the colours of the British suffragette movement. The motto, “More in Common,” is displayed below the shield, and comes from her maiden speech made in Parliament, in which she said, “We are far more united and have far more in common than that which divides us.”

Following the approval by the European Parliament on the Withdrawal Agreement on 29 January 2020, European Parliament President David Sassoli ended his address by referencing Jo Cox’s quote, “More in Common”.