She carried on a long-term love affair with detective writer Dashiell Hammett, who encouraged her to write her first play. The Children's Hour (1934) explored abuse of power, and it ran on Broadway for more than two years. It concerned two women who run a girls' boarding school and are accused falsely by an angry student of carrying on a lesbian affair.
She publicly supported the Spanish leftists against the Franco forces, she was briefly and nominally a member of the Communist Party, but mainly she was fiercely independent, saying it was clear to her she was not suited for any political party. In the 1950s, she was called to testify before the House Committee on Un-American Activities but refused to name names, instead testifying only about herself.
Her best-known play may be The Little Foxes (1939), about siblings struggling for the control of the family business. It is considered a scathing critique of capitalism. It starred Tallulah Bankhead in the premiere performance, and ran for more than four years on Broadway. It has been revived many times and was adapted for film by Hellman in 1941, The film starring Bette Davis was nominated for nine Oscars.
Her play Watch on the Rhine (1941) deals with a family's struggle against the rise of Fascism in Germany before World War II. It, too, was made into a film, also starring Bette Davis. Dashiell Hammett wrote the screenplay.
Hellman's The Lark (1955) is an English adaptation of a French play about Joan of Arc. Her Toys in the Attic (1960) is a family drama set in her native New Orleans. She had no involvement in the film of the same name, which flopped.
She won the National Book Award for her memoir, An Unfinished Woman (1969). Part of her second memoir, Pentimento: A Book of Portraits (1973), was the basis of the Oscar-winning film Julia (1977), starring Jane Fonda and Vanessa Redgrave. Her third memoir, Scoundrel Time (1976), dealt with her experience in testifying before Congress during the McCarthy era. Her short novel, Maybe: A Story (1980), was published as fiction but includes the author, Hammett and other real people. Some critics suggested that it was another installment of her memoirs.
Hellman died in 1984 at her home in Martha's Vineyard. She was 79 years old.