Lennart was born in Brooklyn, New York.^[2] She worked in the MGM mail room in New York, a job she lost when she attempted to organize a union.^[3]

She moved to Los Angeles in 1937,^[3] where she was reported to be working as a secretary to Richard Schayer, then at Metro Studios.^[4] She joined the Communist Party in 1938 but left in August 1939 because "of her opposition to the German-Soviet Nonaggression Treaty".^[5] In 1941, she was promoted from "script-girl to contract writer" at 20th Century Studios.^[6] She rejoined the Community Party in June 1941, as what she called "a passive member".^[5]

Lennart's first script, The Affairs of Martha, an original comedy about the residents of a wealthy community who fear their secrets are about to be revealed in an exposé written by one of their maids, was filmed in 1942 with Spring Byington, Marjorie Main, and Richard Carlson. She was hired by MGM in a full-time position.^[7] This was followed in quick succession by A Stranger in Town (1943), Anchors Aweigh (1945)^[8] and It Happened in Brooklyn (1947).

Lennart left the Communist Party again in Spring 1945.^[9] In 1947, the House Un-American Activities Committee (HUAC) began an investigation into the motion picture industry. Although she was never blacklisted, Lennart, a former member of the Young Communist League, testified to HUAC in 1952 to avoid being blacklisted, and named 23 people.^[10] In 1970, she said that she regretted this decision.^[11]

Lennart's later screen credits include A Life of Her Own, Love Me or Leave Me for which she received an Academy Award nomination in 1955,^[12] Merry Andrew, The Inn of the Sixth Happiness, Please Don't Eat the Daisies,^[13] The Sundowners which also received an Academy Award nomination,^[14] and Two for the Seesaw.

In 1964, Lennart wrote the book for the Broadway musical Funny Girl, based on the life and career of Fanny Brice and her tempestuous relationship with gambler Nick Arnstein. It catapulted Barbra Streisand to fame and earned her a Tony Award nomination. She stated "I wrote the story of Fanny Brice first as a screen play. Vincent J. Donehue, the director, read some pages at my home in Malibu one day and went wild about them. He called Mary Martin and later Ray Stark, and the thing just snowballed. Ray wanted me to do it as a play and I agreed just to please him. Well, not altogether. My vanity entered into it; I didn't want anyone else messing around with my idea."^[8]

In 1966, she won the Laurel Award for Screenwriting Achievement from the Writers Guild of America.^[15] By 1967 she was "commanding $100,000 per picture".^[16]

In 1968, Lennart wrote the screen adaptation of Funny Girl (film) which won her a Writers Guild of America award for Best Screenplay. It proved to be her last work. Three years later, aged 55, she was killed in an automobile crash in Hemet, California.^[17]

Lennart's father was Edward M. Hochdorf, a dentist, and her mother was Victoria Lennart Livingston.^[3] As a child, Lennart caught polio and spent 10 years in leg braces.^[3] Lennart married actor/writer John Harding in Las Vegas, Nevada in 1945. They had two children, Joshua Lennart Harding (December 27, 1947 – August 4, 1971) and Sarah Elizabeth Harding (born November 24, 1951).

• Isobel Lennart at IMDb
• Isobel Lennart at the Internet Broadway Database
• The Isobel Lennart papers at the American Heritage Center