Born June 26, 1970, as the youngest of five children, Hayes has since compared his suburban Chicago upbringing to "being raised by a family of wild dingoes." After high school in Glen Ellyn, Illinois, he attended Illinois State University, where he supported himself as a classical pianist and performed in a pop band. Following his graduation, where he was awarded a degree in performance and conducting, he went to work in the Chicago theater. For several years, he served as the musical director of the Pheasant Run Theater, often acting in productions in addition to writing music for them. He was also a member of the famed Second City improvisational group and performed standup comedy in clubs in Chicago and Los Angeles.
While still living in Chicago, Hayes began finding TV work, notably in the series Silk Stalkings. In 1998, he made his film debut in Billy's Hollywood Screen Kiss, in which he played the title character, a gay photographer in search of love. The film received both a favorable reception at Sundance and a wide art house release. It was while he was at Sundance promoting the movie that Hayes was asked to audition for the part of the straightlaced lawyer Will in the upcoming Will & Grace. Preferring the role of Will's friend Jack, Hayes auditioned for that part instead, and subsuquently became a full fledged star for his portrayal of the beloved character. Following the show's eight year run, Hayes would enjoy a variety of projects, appearing in films like The Bucket List, Soul Men, and The Three Stooges.
After logging several high-profile guest runs on TV shows like Up All Night and Smash, Hayes tried to mount his television comeback with his own show, Sean Saves the World, in 2013. The show only lasted on season, but Hayes quickly rebounded by joining the cast of The Millers in season 2. He also launched a successful production company, executive producing shows like Grimm, Hot in Cleveland and The Soul Man, and created the game show Hollywood Game Night.