She tried unsuccessfully to write novels and worked for 18 months at the Foreign Office writing Cold War propaganda. In 1960 she married Ronald Weldon, an antiques dealer.

During the 1960s Weldon found work doing market research for the London Daily News and writing advertising copy for Ogilvy, Benson, & Mather and other firms. This work paid better and brought her some renown when she coined the popular British slogan, "Go to work on an egg." While decrying advertising as a "shameful business, " Weldon acknowledged that her years as a copywriter forced her to make every word count, an ability that is reflected in her sharply succinct prose style.

According to an interview, Weldon went through psychoanalysis in her early thirties and it was this "dreadfully painful and very interesting" experience that enabled her to try writing fiction again. In the mid-1960s she began writing television plays, which were produced by BBC and one of which, The Fat Woman's Joke, was published in the United States as the novel And the Wife Ran Away. So began her career as a prolific writer of dramas and novels. By 1990 Weldon had written more than 50 scripts for British television, including two episodes of Upstairs, Downstairs, one of which won an award from the Society of Film and Television Arts in 1971. She wrote adaptations for the screen of her own fiction, as well as that of Penelope Mortimer and Elisabeth Bowen. Her five-part dramatization of Jane Austen's Pride and Prejudice was produced on BBC in 1980 after Weldon spent four years completing the adaptation. In 1984 she wrote Letters to Alice on First Reading Jane Austen, a nonfiction work comprised of 16 witty and informative letters to a fictional niece with literary aspirations, explaining the life and times of both Austen and Weldon. Throughout the 1970s and 1980s Weldon wrote plays for television and radio and even the libretto for an operatic version of Ibsen's A Doll's House.

However impressive her other work, it is for her 18 novels that she is best known. One, The Life and Loves of a She-Devil, published in 1984, was serialized on BBC and made into a popular movie in the United States. Her short and fast-paced novels are pastiches of science fiction, economic theory, surreal imagery, psychological insight, and political satire. The reader turns the pages of a Weldon novel not so much to discover what its characters will do next, but rather to learn what brilliant comic moves Weldon herself will engineer to drive the story. Most of her work is translated into many languages and distributed around the world.

Weldon's childhood experience in a largely female world as a child of divorce raised by a working mother, as well as her own later struggle as a single mother, are reflected in the characters who people her fictional worlds. However, her later life was very different. Married to Ronald Weldon for over thirty years, they raised a family of sons. Weldon's oldest son, Nicholas, was a jazz musician as well as a chef and Weldon's business manager. Daniel (1963) was a filmmaker, Thomas (1970) was described by his mother as a "practicing punk," and Samuel (1977) lived with his parents in the Somerset town of Shepton Mallet. Weldon herself commuted two days a week to a house in Kentish Town, London. In 1997 Weldon provided yet another unique profile on women in Wicked Women, a collection of short stories taking place in the 1990s.

Weldon's writing is reviewed in American newspapers and periodicals such as The New York Times, Los Angeles Times, New Yorker, The Washington Post, and Village Voice. For interviews see Marjorie Williams in The Washington Post (April 24, 1988) and Eden Ross Lipson in Lear's (January 1990). Brigitte Salzmann-Brunner in Amanuenses to the Present: Protagonists in the Fiction of Penelope Mortimer, Margaret Drabble, and Fay Weldon (1988) looks at Weldon's work in the context of that of her peers. See also Carolyn Nizzi Warmbold, "Books: Reviews and Opinion: In Brief: 'Wicked Women' by Fay Weldon, "The Atlantic Journal and Constitution (June 22, 1997).