In 1939, Romilly and Mitford emigrated to the United States. They travelled around, working odd jobs, perpetually short of cash. At the outset of World War II, Romilly enlisted in the Royal Canadian Air Force; Mitford was living in Washington D.C. and considered joining him once he was posted to England. She gave birth to another daughter, Constancia ("the Donk" or "Dinky") Romilly on 9 February 1941. Her husband went missing in action on 30 November 1941, on his way back from a bombing raid over Nazi Germany. She took months to accept that he was dead.
In 1956, Mitford published (stenciled) a pamphlet, "Lifeitselfmanship or How to Become a Precisely-Because Man". In response to Noblesse Oblige, the book her sister Nancy co-wrote and edited on the class distinctions in British English, popularising the phrases "U and non-U English" (upper class and non-upper class), Jessica described L and non-L (Left and non-Left) English, mocking the clichés used by her comrades in the all-out class struggle. (The title alludes to Stephen Potter's satirical series of books that included Lifemanship.)
Feeling that in the current political climate they could do more for social justice outside the Party, and disillusioned by the development of Communism in the Soviet Union, Mitford and Treuhaft resigned from it in late 1958. Evidently she had to become a United States citizen or she would have been unceremoniously deported, regardless of her husband's citizenship.
In 1960 Mitford published her first book Hons and Rebels (American title: Daughters and Rebels), a memoir covering her youth in the Redesdale household.
Through his work with unions and death benefits, Treuhaft became interested in the funeral industry and persuaded Mitford to write an investigative article on the subject. Though the article, "Saint Peter Don't You Call Me" published in Frontier magazine, was not widely disseminated, it caught considerable attention when Mitford appeared on a local television broadcast with two industry representatives. Convinced of public interest, she wrote The American Way of Death, which was published in 1963. In the book Mitford harshly criticized the industry for using unscrupulous business practices to take advantage of grieving families. The book became a major bestseller and led to Congressional hearings on the funeral industry. The book was one of the inspirations for filmmaker Tony Richardson's 1965 film The Loved One, which was based on Evelyn Waugh's short satirical 1948 novel of the same name, tellingly subtitled "An Anglo-American Tragedy".
After The American Way of Death Mitford developed a name for her investigative journalism. In 1970, she published an article in the Atlantic Monthly "Let Us Now Appraise Famous Writers", an exposé of the Famous Writers School, a correspondence course of questionable business practices founded by Bennett Cerf. She published The Trial of Dr. Spock, the Rev. William Sloane Coffin, Jr., Michael Ferber, Mitchel Goodman, and Marcus Raskin, an account of the five men's 1970 trial on charges of conspiracy to violate the draft laws, followed by a harsh critique of the American prison system entitled Kind and Usual Punishment: The Prison Business (1973), an allusion to the phrase "cruel and unusual punishment".
Mitford was a distinguished professor for the fall semester 1973 at San Jose State University where she taught a course called "The American Way", which analyzed how the Watergate gang cut their teeth in the earlier witch-hunt against the left in the McCarthy era. Due to disagreements with the dean over her taking a loyalty oath and submitting to fingerprinting, the campus was thrown into protests and she was forced to go to court to remain able to teach.
Mitford's second memoir, A Fine Old Conflict (1977), comically describes her experiences joining and eventually leaving the Communist Party USA. Mitford titled the book after what, in her youth, she thought were the lyrics to the Communist anthem, "The Internationale", which actually are "Tis the final conflict". Mitford recounts how she was invited to join the Communist Party by her co-worker Dobby, to whom she responded "We thought you'd never ask!" She bristled against the conservative structure in the CP, at one point upsetting the women's caucus by printing a poster with "Girls! Girls! Girls!" to draw people to an event. She mercilessly teased an elder Communist about his paranoia when he wrote out the name of a town where she could get chickens donated from "loyal party members" for a fundraiser. When he wrote Petaluma on a scrap of paper to avoid being overheard by possible bugs, she asked in jest how the chickens should be prepared, and wrote, "Fried or broiled".
In addition to writing and activism, Mitford tried her hand at music as singer for "Decca and the Dectones," a cowbell and kazoo orchestra. She performed at numerous benefits and opened for Cyndi Lauper on the roof of the Virgin Records store in San Francisco. She recorded two short albums: one contains her rendition of "Maxwell's Silver Hammer" and "Grace Darling," and the other, two duets with friend and poet Maya Angelou.
Her last work was an update entitled The American way of Death Revisited.
Mitford died of lung cancer at age 78. Her widower survived her by five years. Their surviving daughter had continued the activist tradition by working for the Student Nonviolent Coordinating Committee. She had two children with James Forman, its African American director, and eventually became an emergency room nurse. Mitford's surviving son, Benjamin, was estranged from his family for some time and developed bipolar disorder (manic depression), but later became a piano tuner and uses his skills to ship pianos to Cuba with the slogan, "Send a piana to Havana."
Rowling stated in 2002, "My most influential writer, without a doubt, is Jessica Mitford. When my great-aunt gave me Hons and Rebels when I was 14, she instantly became my heroine. She ran away from home to fight in the Spanish Civil War, taking with her a camera that she had charged to her father's account. I wished I'd had the nerve to do something like that. I love the way she never outgrew some of her adolescent traits, remaining true to her politics – she was a self-taught socialist – throughout her life. I think I've read everything she wrote. I even called my daughter [Jessica Rowling Arantes] after her.