Rayner was born on 28 March 1980 in Stockport, Greater Manchester. She attended Avondale School in Stockport, leaving the school aged 16 after becoming pregnant, and did not obtain any qualifications. She later studied part-time at Stockport College, learning British Sign Language, and qualifying as a social care worker.
After leaving college, Rayner worked for Stockport Metropolitan Borough Council as a care worker for a number of years. During this time, she was also elected as a trade union representative for Unison. She was later elected as convenor of Unison North West, becoming the union's most senior official in the region.
The Guardian featured a lengthy profile of Rayner in 2012, as part of an article on a trade union officer's working life.
In 2014, Rayner was selected to become Labour's candidate for the seat of Ashton-under-Lyne, after the retirement of David Heyes. She won the seat at the 2015 general election, increasing both the Labour majority and its share of the vote in the constituency. She delivered her maiden speech in the House of Commons on 3 June 2015. In the 2017 general election held on Thursday 8 June 2017, Rayner successfully defended her Ashton-under-Lyne seat, increasing both the Labour majority and vote share. Rayner held her Ashton-under-Lyne seat a third time after winning the constituency in the general election held on Thursday 12th December 2019 but with a smaller majority, and a reduced vote share (-12%).
Rayner nominated Andy Burnham in the 2015 Labour leadership election, but the following year became one of just 18 MPs to back the incumbent Jeremy Corbyn against challenger Owen Smith in the 2016 leadership election.
On 1 July 2016, after a series of resignations from the Shadow Cabinet, Corbyn appointed Rayner to the position of Shadow Secretary of State for Education.
In the 2019 General Election Rayner successfully defended her Ashton-under-Lyne seat being returned as Member of Parliament for the third time in 5 years, but with a smaller majority than in 2017 and 2015.
In a 2017 interview discussing her political beliefs, Rayner highlighted her pragmatism, described herself as being part of the "soft left" of the Labour Party. As shadow education secretary, Rayner has championed the policy of a "National Education Service", which would be modelled along similar lines to the UK National Health Service, as well as promoting an increase in funding to early years education. She has previously written that decreasing teachers' salaries would lead to shortages in those joining the profession.
In 2010, she married Mark Rayner, a Unison official. She has three sons: Ryan, Jimmy and Charlie. Charlie was born prematurely at 23 weeks and Rayner says that the care he received demonstrated the importance of the NHS to her. Rayner lives in her constituency of Ashton-under-Lyne with her family. She became a grandmother in November 2017.
In an interview with Evan Davis of the BBC in 2018, Rayner said that her mother had been unable to read or write; a repeat of part of a tribute she made to her mother in 2016.
In March 2019, Rayner revealed that she had fitted panic buttons at her home after rape and death threats were sent to her a few weeks earlier.