After unsuccessfully fighting Harwich in 1964, Winnick was elected in 1966 as the MP for Croydon South, defeating incumbent Richard Thompson. He lost the seat to Thompson in 1970. After completing a diploma in social administration at the London School of Economics, he stood again unsuccessfully in Croydon Central in October 1974 and was returned for Walsall North in 1979. Winnick is generally regarded as being on the left of the Labour Party and has a strong commitment to human rights. That commitment made him a strong voice in the House of Commons against both the Taliban and Saddam Hussein and he supported the 2003 invasion of Iraq. He was a member of the British-Irish Parliamentary body from its formation in 1990, and British co-chair, 1997–2005. On 9 November 2005, Winnick's amendment to a government bill on detention of terrorist suspects without trial, proposing that the maximum period of detention should be 28 days rather than 90, passed in the House of Commons by 323 votes to 290, shortly after the government's 90-day proposal was defeated by 322 to 291. This was Tony Blair's first defeat in the House of Commons on a whipped vote, after serving nearly nine years as Prime Minister, and may come to be seen as a critical moment of his term in office. In January 2009, he urged the Communities Minister to deplore the fact that Richard Williamson, a British-born bishop and Holocaust denier, had been brought back into the fold by the Vatican. Winnick played a prominent role in the campaign to force the resignation of the Speaker of the House of Commons, Michael Martin. This followed controversy from May 2009 concerning MPs' disclosure of expenses. At the 2010 general election, Walsall North was one of the most closely contested seats at the election, with Winnick being re-elected with a significantly reduced majority of 990 votes, compared to 6,640 votes at the previous election five years earlier. In 2015, however, Winnick gained a comfortable majority of 1,937, despite the Labour Party incurring a net loss of seats. In his victory speech, he criticised the way in which his Conservative opponent had conducted their election campaign. In 2017 he was defeated by Conservative Party candidate Eddie Hughes by 2,601 votes.
