Born on February 2, 1963, at the Washington Hospital Center in Washington, D.C., Cassidy grew up in Oxon Hill, Maryland, and later Bowie, Maryland. She was the third of four children. Her father, Hugh Cassidy, is a teacher, sculptor, musician, former army medic, and world champion powerlifter of Irish and Scottish descent, while her mother, Barbara ("née" Kratzer), is a German horticulturist from Bad Kreuznach. From an early age, Cassidy displayed interest in art and music. When she was nine, her father began teaching her to play the guitar, and she began to play and sing at family gatherings.