Mendenhall was born in Oceanside, California. He has a younger sister, Marissa. Mendenhall began his acting career at the age of four. In 1979, he auditioned for and won the role of Mike Webber on the soap opera General Hospital. He remained with the series until 1986. During that time, Mendenhall started acting in feature films. Mendenhall made his film debut in the 1983 space Westerm, Space Raiders, produced by Roger Corman. Notable television appearances during the 80s also included playing Elaine Nardo's son on Taxi on three episodes, and playing a 12-year-old drug dealer in “The Reporter”, a very special episode of the sitcom Diff'rent Strokes. The episode featured a guest appearance by then-First Lady Nancy Reagan.