Kael was known for her "witty, biting, highly opinionated, and sharply focused" reviews, her opinions often contrary to those of her contemporaries. She is often regarded as the most influential American film critic of her day.

She left a lasting impression on many major critics, including Armond White, whose reviews are similarly non-conformist, and Roger Ebert, who has said that Kael "had a more positive influence on the climate for film in America than any other single person over the last three decades." Owen Gleiberman said she "was more than a great critic. She re-invented the form, and pioneered an entire aesthetic of writing. She was like the Elvis or the Beatles of film criticism."

Kael was born on a chicken farm in Petaluma, California, to Isaac Paul Kael and Judith Friedman Kael, Jewish immigrants from Poland. Her parents lost their farm when Kael was eight and the family moved to San Francisco, California. She matriculated at the University of California, Berkeley in 1936; she studied philosophy, literature and the arts but dropped out in 1940 before completing her degree. Nevertheless, Kael intended to go on to law school but fell in with a group of artists and moved to New York City with the poet Robert Horan.

Three years later, Kael returned to San Francisco and "led a bohemian life," marrying and divorcing three times, writing plays, and working in experimental film. In 1948, Kael and filmmaker James Broughton had a daughter, Gina, whom Kael would raise alone. Gina had a serious illness through much of her childhood, and to support her, Kael worked a series of menial jobs, such as cook and seamstress, along with stints as an ad-copy writer. In 1953, the editor of City Lights magazine overheard Kael arguing about movies in a coffeeshop with a friend and asked her to review Charlie Chaplin's Limelight. Kael memorably dubbed the movie "slimelight," and began publishing film criticism regularly in magazines.

Even these early reviews were notable for their informality and lack of pretension; Kael later explained, "I worked to loosen my style—to get away from the term-paper pomposity that we learn at college. I wanted the sentences to breathe, to have the sound of a human voice." Kael disparaged the supposed critic's ideal of objectivity, referring to it as "saphead objectivity," and incorporated aspects of autobiography into her criticism. In a review of Vittorio De Sica's 1946 neorealist Shoeshine (Sciuscià) that has been ranked among her most memorable, Kael described seeing the film

Kael broadcast many of her early reviews on the alternative public radio station KPFA in Berkeley, and gained further local-celebrity status as Berkeley Cinema Guild manager from 1955 to 1960. As manager of a two-screen theater, Kael programmed the films that were shown "unapologetically repeat[ing] her favorites until they also became audience favorites." She also wrote "pungent" capsule reviews of the movies, which her patrons began collecting.

Kael continued to juggle writing with other work until she received an offer to publish a book of her criticism. Published in 1965 as I Lost It at the Movies, the collection sold 150,000 paperback copies and was a surprise bestseller. Coinciding with a job at the high-circulation women's magazine McCall's, Kael (as Newsweek put it in a 1966 profile) "went mass."

During the same year, she wrote a blistering review of the phenomenally popular The Sound of Music in McCall's. After mentioning that some of the press had dubbed it "The Sound of Money," Kael called the film's message a "sugarcoated lie that people seem to want to eat." Although, according to legend, this review led to her being fired from McCall's (The New York Times printed as much in Kael's obituary), both Kael and the magazine's editor, Robert Stein, denied this. According to Stein, "I [fired her] months later after she kept panning every commercial movie from Lawrence of Arabia and Dr. Zhivago to The Pawnbroker and A Hard Day's Night."

Her dismissal from McCall's led to a stint from 1966 to 1967 at The New Republic, whose editors continually altered Kael's writing without permission. In October 1967, Kael wrote a lengthy essay on “Bonnie and Clyde”, which he magazine declined to publish. William Shawn of The New Yorker got hold of the piece and ran it in the New Yorker issue of October 21. Kaels review raved about the then controversial film Bonnie and Clyde. According to critic David Thomson, "she was right about a film that had bewildered many other critics." A few months after the essay ran, Kael quit the Republic "in despair," Kael was asked by Shawn to join The New Yorker staff as one of its two film critics (she alternated every six months with Penelope Gilliatt until 1979, after which she became sole film critic),

Initially, many considered her colloquial, brash writing style an odd fit with the sophisticated and genteel New Yorker. Kael remembered "getting a letter from an eminent New Yorker writer suggesting that I was trampling through the pages of the magazine with cowboy boots covered with dung." During her tenure at the New Yorker, however, she took advantage of a forum that permitted her to write at length and with presumably minimal editorial interference, and Kael achieved her greatest prominence; by 1968, Time magazine was referring to her as "one of the country's top movie critics." Kael noted that during this period her reviews were so interesting because the movies were so compelling.