Leoni attended Brearley School and The Putney School. She attended but did not complete studies at Sarah Lawrence College. In 1988, she was cast as one of the stars of Angels 89, an updated version of the 1970s show Charlie's Angels. In May 1988, Life magazine ran a story on the casting of this new series. After production delays, the show never aired. The following year, Leoni starred as "Lisa DiNapoli" in the TV daytime drama Santa Barbara. She went on to appear in the women's-baseball film A League of Their Own (1992) and later that year starred with Corey Parker in the single-season Fox sitcom Flying Blind, playing a beautiful, eccentric, downtown-Manhattan libertine who falls in love with an ordinary Long Island college graduate. In February 1995, she appeared in the sitcom Frasier as the fiance of Sam Malone, portrayed by guest star Ted Danson, in a crossover from Cheers. That same year, Leoni landed the lead role in the sitcom The Naked Truth, playing Nora Wilde, a tabloid news journalist. The show ran through 1998.