McPhail was born and raised in Columbus, Ohio, United States. Later, she moved to Toronto, Canada, where she attended the Etobicoke School of the Arts. At age 14, she was cast in her first major role as Annie Edison in the children's television series The Edison Twins (1982). She met her husband, Reed Diamond, on the set of Scared Silent (2002) and they have been married since 2004. McPhail and her husband are both members of the rock band "Chuck Valiant;" McPhail is the lead singer and Diamond plays guitar.
