Born in Wimbledon, now part of London, Mavis left school at 16 to do office work with Editions Alecto, a Kensington publishing company. She later moved to the firms gallery in Albemarle Street, where she met artists such as David Hockney, Allen Jones, Patrick Caulfield and Gillian Ayres. In 1969 she married a childhood sweetheart, Chris Cheek, a physicist, whom she had met at a meeting of the Young Communist League in New Malden, but they separated three years later. Later she lived for eleven years with the artist Basil Beattie. She returned to education in 1976, doing a two-year arts course at Hillcroft College, a further education college for women.

Although Cheek had planned to take a degree course, she turned instead to fiction writing while her daughter, Bella Beattie, was a child. She moved from London to Aldbourne in the Wiltshire countryside in 2003, but as she explained to a newspaper, Life in the city was a comparative breeze. Life in the country is tough, a little bit dangerous and not for wimps.

Cheek has been involved with the Marlborough LitFest, and also teaches creative writing. This has included voluntary work at Holloway and Erlstoke prisons. As she described in an article: What I see [at Erlstoke] is reflected in my own experience. Bright, overlooked, unconfident men who are suddenly given the opportunity to learn grow wings, and dare to fail. It helps to be able to tell them that I, too, was once designated thick by a very silly [education] system. My prisoners have written some brilliant stuff, and perhaps it gives them back some self-esteem.