Gellar was born in New York City to Jewish-American parents Arthur and Rosellen. After her parents divorced in 1984, she was brought up by her mother. She refuses to acknowledge any relationship with her father saying "Just because you donate sperm does not make you a father." Arthur died on October 9, 2001 of unknown causes.

She has been acting since the age of four, when she did a commercial for Burger King. Her best friend as a child was Melissa Joan Hart, who later was the star of the television series Sabrina, the Teenage Witch. Gellar's major break came in 1992 in the teen soap opera Swan's Crossing. From there, she moved on to the long-running soap opera All My Children, where she played the conniving character Kendall Hart Lang, the long-lost daughter of character Erica Kane (played by Susan Lucci). In 1995, at the age of 18, she won a Daytime Emmy Award for Outstanding Younger Leading Actress in a Drama Series. Gellar left All My Children in 1995 and landed the lead in the tv series Buffy the Vampire Slayer, created by Joss Whedon, as a teenager burdened with the responsibility of fighting a number of mystical foes, with the aid of a group of friends and her Watcher (a teacher of sorts). The show was well received by critics and audiences alike, and throughout its seven seasons, Buffy, and Gellar along with her, became cult icons in the United States and the UK. While continuing in that role, she attempted to capitalize on her television fame in order to create a career for herself in motion pictures, with intermittent commercial success. After roles in the popular thrillers I Know What You Did Last Summer and Scream 2, Gellar starred in the film Simply Irresistible. This film, rumored to be the last film ever watched by critic Gene Siskel, featured a magical crab and borrowed heavily from Like Water for Chocolate. Gellar's next film was the steamy hit Cruel Intentions, a modern-day retelling of Les Liaisons Dangereuses; this movie gained much exposure due to a famous girl on girl kissing scene between Sarah and Selma Blair. Also in 1999, she made an appearance in the Stone Temple Pilots music video Sour Girl. She then went on to play a lead role in Harvard Man.

On September 1, 2002, Gellar and teen-movie actor Freddie Prinze Jr. were married in Mexico. Gellar found greater box office success playing Daphne in Scooby-Doo, a live-action adaption of the cartoon series. Despite being universally panned by critics, the movie was highly popular with audiences. Gellar also appeared in the movie's sequel, Scooby-Doo 2: Monsters Unleashed (2004). The year of 2004 was marked by a point of controversy in Gellar's career, as the president of her own talent agency, David Wirtschafter, indicated that he believed her movie career was floundering as of fall 2004, telling The New Yorker that the success of her low-budget film The Grudge (a remake of the Japanese horror film Ju-on: The Grudge) "takes our client Sarah Michelle Gellar, who now is nothing at all, and it makes her a star, potentially. Suddenly, the Sarah Michelle Gellar space is meaningful." The remark led to Gellar abandoning the agency. Her next film project will be Southland Tales, a film by Donnie Darko director Richard Kelly. [1] Gellar also sang several of the songs during the Buffy the Vampire Slayer musical episode "Once More, with Feeling," which spawned an original cast album.