A graduate of Warwick University, Cruddas was first elected to Parliament in 2001. Having been critical of many aspects of the Blair Government, Cruddas stood for the Deputy Leadership of the Labour Party in 2007, being eliminated in the penultimate round of the contest. Unlike the other five candidates, he openly stated that he did not wish to become Deputy Prime Minister.
He won the most votes in the first round of voting, obtaining 19.39% of the vote from both party members and party-affiliated organisations, and it is thought that the second-choice votes of the Cruddas supporters contributed to Harriet Harman's eventual victory. After his campaign, he was offered a position in the Cabinet]] by Prime Minister Gordon Brown, which he turned down.
Despite being touted by some media sources as a potential candidate for the Leadership of the Labour Party in the future, he ruled himself out of the 2010 leadership election, saying that he did not want the job but instead wanted to influence policy.
In 1989, he became a policy officer for the Labour Party before being appointed Senior Assistant to Labour Party General Secretary Larry Whitty in 1994, remaining in that position when Tom Sawyer became General Secretary that same year. After the 1997 election, he was employed as Deputy Political Secretary to new Prime Minister Tony Blair. His main role was to be a liaison between the Prime Minister and the trade unions, with whom Blair had often had a difficult relationship. In this role, he also worked heavily on the introduction of the minimum wage.
Cruddas was selected to be the Prospective Parliamentary Candidate for the safe Labour seat of Dagenham in 2000, after incumbent MP Judith Church announced that she would be retiring. He was elected as the new MP for Dagenham one year later at the 2001 election, with a majority of 8,693.
From the backbenches, Cruddas quickly became a vocal critic of the government for what he saw as their ignoring of their traditional, working-class support in a bid to woo middle-class voters. He rebelled against the government on a number of occasions, including on the introduction of Tuition fees, the legislation on asylum seekers, the introduction of trust schools, proposals to renew the UK Trident nuclear weapons system, and foundation trusts.
Cruddas was re-elected at the 2005 election, but his Dagenham constituency was abolished in boundary changes for the 2010 election. Cruddas chose to fight the newly created seat of Dagenham and Rainham, which was notionally marginal. He won the seat by 2,630 votes in a close-run election campaign, which was a seat that the British National Party had heavily targeted. This resulted in a large number of anti-fascist organisations not affiliated to the Labour Party, such as Hope not Hate, campaigning for Cruddas to resist the BNP. After winning re-election, he took up a part-time position teaching Labour history at University College, Oxford from 2010–12.
On 27 September 2006, Cruddas announced his intention to stand to become Deputy Leader of the Labour Party once the incumbent, John Prescott, stood down. He said he did not want to be Deputy Prime Minister, but instead wished to act as a "transmission belt" with the grassroots of the party. In interviews, Cruddas also said that he did not want the "trappings or baubles" that would potentially come with the job of Deputy Prime Minister, such as use of the Dorneywood weekend country residence. Cruddas accrued nominations from 49 MPs and received strong union backing, including Amicus and the Transport and General Workers' Union. On 24 June 2007, it was announced that Harriet Harman had won the election, although Cruddas gained the highest proportion of votes in the first round. He was ultimately eliminated in the fourth round of voting, coming third behind Harman and Alan Johnson. He had secured the highest number of votes from members of affiliated organisation in every round before his elimination.
On 15 May 2012, Labour Leader Edward Miliband offered Cruddas a position in his Shadow Cabinet as Labour's Policy Coordinator, with a view to crafting Labour's manifesto for the 2015 general election. Cruddas accepted the offer, saying that it had always been his wish to influence policy.
The Future of Work Commission was announced at the 2016 Labour Party Annual Conference in Liverpool. The goal of the commission is to make a set of achievable policy recommendations, which will be delivered in a report in September 2017 at Labour Annual Conference in Brighton. Jon Cruddas MP is one of the Commissioners working on the project.