Garth was born in the farming community of Urbana, Illinois, and spent her early childhood on the family's 25-acre horse farm in Arcola, Illinois. With her parents and six siblings, Garth moved to Phoenix, Arizona, when she was just 13, and it was there that she met her manager at a local beauty pageant. She then convinced her parents she needed to move to Los Angeles to pursue an acting career. Once in Los Angeles, Garth quickly landed a role in her first television series, A Brand New Life starring Barbara Eden. At 17, Garth dazzled legendary producer Aaron Spelling who hired her to play spoiled rich girl Kelly Taylor even before she got back to her car. In 1996, Garth executive produced and starred in the television movie An Unfinished Affair, where she met her future husband, actor Peter Facinelli. Garth's other credits include the television movies Without Consent, Loss of Innocence, Danielle Steel's Star, Darren Star's television series The Street and the made-for-tv movie NBC's Secret Santa.