Sibbett started her acting career as Jane Wilson on the NBC soap opera "Santa Barbara" in 1986-87, for which she was nominated for a Best Newcomer Soap Opera Digest Awards. In 1989, she won the role of Laurie Parr on the CBS comedy "The Famous Teddy Z", co-starring with Jon Cryer and Alex Rocco. The series lasted one season. In 1991, Sibbett was cast as status-conscious bombshell Heddy Newman on the Fox sitcom "Herman's Head", which quickly gained a cult following on the young broadcast network and lasted three seasons. Beginning in 1994, she played the occasional role of Carol Willick on "Friends", a part-time stint that lasted until the end of the show's seventh season in 2001. During her "Friends" association, Sibbett had regular roles on the short-lived CBS sitcom "If Not for You" (1995), playing the jilted fiancée of her former "Herman's Head" co-star Hank Azaria, and in the second season of The WB's "" (1997–98), playing school headmaster Dr. Katherine Emerson.