Quilter was born out of wedlock, and grew up without her father, John william G. Klein (1899-1973), initially in Willesden and later in Mid Holmwood near Dorking. She was bullied at school due to being born illegitimately, and suffered from low self-esteem throughout her childhood and teenage years. She left school at 15 and in 1964 she married Robert Maxted, and lived in Dorking. She had to nurse her terminally ill mother for more than ten years, and began her porn career to pay for her care. She had decided that she wanted to be a fashion model but at 4 feet 11 inches she was not tall enough. Instead she became a glamour model in the late 1960s.

Soon after becoming a glamour model, she met the glamour Photographer and pornographer John Jesnor Lindsay, who offered to photograph her for softcore magazines. She became one of his most popular Models and began appearing in 8 mm hardcore porn loops which sold well in Europe. One of her first films was Miss Bohrloch in 1970. Miss Bohrloch won the Golden Phallus Award at the Wet Dream Festival held in November 1970 in Amsterdam. Maxted starred in around twenty short hardcore films for John Lindsay, although only five (Miss Bohrloch, Oral Connection, Betrayed, Oh Nurse and Special Assignment) have so far resurfaced. She then returned to modelling for British pornographic magazines such as Knave and Men Only. She also appeared in softcore short films by Russell Gay (Response, 1974), Mountain Films (Love Games, Wild Lovers) and Harrison Marks (Sex is My Business, c. 1974).

In February 1974, Maureen O’Malley, her co-star in Sex is My Business introduced her to adult magazine publisher David Sullivan. Although she was still married, the pair became lovers. He rebranded her as Mary Millington. In her first appearance in Sullivan's Whitehouse magazine, he claimed that she was the bisexual nymphomaniac sister of the magazine's Editor Doreen Millington, and so gave Mary her new stage name. She became well-known thanks to her appearances in Sullivan's porn magazines such as Whitehouse and Private. She soon became the most popular model in any of Sullivan's magazines. In November 1977 magistrates acquitted her and Sullivan following prosecution under the Obscene Publications Acts.

Millington has been described as one of the "two hottest British sex film stars of the seventies", the other being Fiona Richmond. David Sullivan described her as "the only really uninhibited, natural sex symbol that Britain ever produced and who believed in what she did". Between 1975 and 1982 there was always at least one of Millington's films playing in London's West End.

She was buried at St Mary Magdalene Church, in South Holmwood, Surrey marked by a grey granite tombstone which bears her married name. She is buried in the same grave as her mother, Joan Quilter, who died in 1976.

A nightclub in Liverpool is named after her. She is commemorated in a blue plaque on the site of the former Moulin Cinema in Great Windmill Street, Soho for her appearance in Come Play with Me. The film ran there continuously for 201 weeks, from April 1977 to March 1981, and is listed in the Guinness Book Of World Records as the longest-running British film. However, the validity of this record and the blue plaque is called into question in the second edition of David McGillivray’s book Doing Rude Things which states that "film Historian Allen Eyles has proven conclusively that Come Play With Me ran for 165 not 201 weeks. It is not Britain’s longest running film – that is South Pacific, which ran... a total of four years and twenty-two weeks. Questions should be asked in parliament about the deception I may have instigated."

In 1978 she was approached to appear in a hardcore porn film called Love is Beautiful, to have been directed by Gerard Damiano. However, despite Millington and Damiano being pictured together at that year's Cannes Film Festival, the movie (meant to have been produced by David Grant’s Oppidan Films) never materialized. Potential co-stars may have included Harry Reems, Gloria Brittain and Lisa Taylor. That same year she turned 33, and found herself being replaced by younger Models in Sullivan's magazines. In March 1978 she ceased to work in Sullivan's Whitehouse sex shop in Norbury and opened her own in Tooting called Mary Millington's International Sex Centre. She began to spend more time working in her own sex shop, selling illegal material. The shop was raided by the police on numerous occasions, and she claimed the police threatened her and forced her to pay protection money. In the past she had publicly criticised police raids on sex shops and published the addresses and telephone numbers of Scotland Yard, the Director of Public Prosecutions and Members of Parliament in her magazines.

Millington committed suicide at age 33, by an overdose of tricyclic antidepressant anafranil, paracetamol and alcohol at her home in Walton-on-the-Hill, Surrey. Her husband found her dead in her bed on 19 August 1979. She left four suicide notes which were found near her body. In one of them she had written, "The police have framed me yet again. They frighten me so much. I can't face the thought of prison... The Nazi tax man has finished me as well." In another note, to her solicitor Michael Kaye (partly published in Private magazine no 59), Millington wrote "the police have killed me with their threats…the police have made my life a misery with frame ups. The tax man has hounded me so much- I will be made bankrupt, he mustn't get anything of his £200,000 demands. He is a religious maniac." In another note, to David Sullivan, she wrote: "please print in your magazines how much I want porn to be legalised, but the police have beaten me".

A posthumous exploitation film about her life was released in London in October 1980, entitled Mary Millington's True Blue Confessions. In 1996, Channel Four screened a tribute to her entitled Sex and Fame: The Mary Millington Story, featuring an interview with David Sullivan.

In 2004 Millington’s prominence was recognized by her inclusion in the Oxford Dictionary of National Biography, edited by Colin Matthew and Brian Harrison. Her entry was written by Richard Davenport-Hines.

In 2008 an exhibition of the work of the late glamour Photographer Fred Grierson was held in London, which included several little-seen pictures of Millington taken by Grierson at June Palmer's Strobe Studios in the early 1970s.

In late 2009, an 8 mm copy of one of her early John Lindsay short films Special Assignment resurfaced. Unseen since the early 1970s, it was subsequently transferred to DVD. Two years later in 2011, Wild Lovers, another 8 mm film starring Millington, was also traced and transferred from 8 mm to DVD.

Twenty years after her death, the author and film Historian Simon Sheridan put Millington's life into context in the biography Come Play with Me: The Life and Films of Mary Millington. Further information about her career can be found in Sheridan's follow-up book Keeping the British End Up: Four Decades of Saucy Cinema, the fourth edition of which was published in April 2011.

The film received its world premiere at London’s Regent Street Cinema on 7 April 2016. A DVD of the film was released in the UK on 2 May 2016.

In 2009, Millington was referenced in an episode of EastEnders (broadcast Thursday 18/06/09) when Aunt Sal (Anna Karen), after hearing Ronnie Mitchell (Samantha Janus) had been caught in a compromising position on the Queen Vic’s floor, greeted Ronnie with "ooh, here she is, Mary Millington herself".

* David Sullivan’s magazines were often undated, as such the only way of dating them is by which Sullivan produced films were being promoted inside the magazines, i.e. a Sullivan magazine which promotes Come Play With Me would be from 1976/1977, ones promoting The Playbirds would be circa 1978, and ones promoting Confessions from the David Galaxy Affair from 1979.