Her TV career also began in earnest in the 90s. Gellar played the young Jacqueline Bouvier in the NBC miniseries "A Woman Named Jackie" in 1991, and was one of the adolescents in a small wealthy suburb on the short-lived syndicated soap opera "Swan's Crossing" (1992). For two years (1993-1995), she played the role of the scheming Kendall Hart on ABC's "All My Children". Her character was supposed to be the child born to Erica Kane (Susan Lucci) after a teen-age rape who arrived in town determined to make Erica pay dearly for having given her up for adoption at birth. Gellar was twice-nominated for an Emmy and shortly after winning in 1995, it was announced she would be leaving the show. Rumors swirled about the young actress and her relationship with the veteran Lucci. (Gellar was quoted by TV GUIDE as saying "We didn't have a perfect working relationship.")

Relocating to California, Gellar won the role of Dyan Cannon's spoiled daughter in the Disney ABC TV-movie movie "Beverly Hills Family Robinson" (1997). She then landed the leading role as Buffy, based on the 1992 feature. Playing a high school teen who is "the chosen one" of her generation to destroy vampires but who also would like to have a normal adolescence, Gellar won critical praise for her comic abilities and her physicality. The series and its star garnered a rabid fan following and continually won over critics, and "Buffy" was able to maintain both high standards and ratings even after switching networks from The WB to UPN for its final two seasons. Rumors swirled that Gellar may have had some behind-the-scenes friction on the show, but it seems likely that she will re-team with creator Joss Whedon and re-assume the "Buffy" role once again at some point in the future (she was nearly tapped for one of the concluding episodes of the spin-off "Angel" but producers feared her star presence would overwhelm the show's internal closure).

Gellar also landed a major role alongside rising stars Ryan Phillippe, future husband-to-be Freddie Prinze Jr and Jennifer Love Hewitt in "I Know What You Did Last Summer" (1997) and a smaller part as one of Neve Campbell's college chums in "Scream 2" (also 1997), both from the pen of screenwriter Kevin Williamson. While she floundered in the romantic comedy "Simply Irresistable" (the film wasn't), Gellar shone as a teenage version of the Marquise de Merteuil in "Cruel Intentions" (both 1997), the surprisingly adept resetting of the French classic "Les Liaisons Dangereuse" to tony Manhattan. The actress next graced the big screen in "Harvard Man" (2001), a sexy cheerleader and Mafia princess who seduces the title character. Geller then reunited with Prinze to co-star in the live-action version of the popular cartoon "Scooby Doo" (2002), an ironic in-joke as the crimefighters on her TV series were nicknamed "the Scooby gang". She also returned for the 2003 sequel "Scooby Doo 2: Monsters Unleashed."

Further attempting to break out as a leading lady in feature films, Gellar used her horror-project cred to help fuel "The Grudge" (2004), an enigmatic thriller that cast her as an American nurse in Tokyo who is exposed to a mystery virus, one that locks a person in a powerful rage before claiming their life and spreading to another victim. The film was remade by director Takashi Shimizu from his original Japanese version and despite a fragmented narrative that left many critics and moviegoers scratching their heads, the combination of Gellar's presence and a strategic near-Halloween release gave the film an immensively successful debut.