Born Marlene Aloisio in Detroit, Michigan, Willoughby chose the surname "Willoughby" for her professional work after someone called her "willowy" due to her lithe, 5'-10" frame. She followed her older sister, Jacqueline Carol, in pursuing a career in show business. In 1969, she appeared in the controversial Lennox Raphael Off-Off-Broadway play Che! Other theater roles followed including the Obie Award winning Dracula Sabbat, Fuck Mother and Keepers of the Hippo Horn. She also landed parts in mainstream fare such as No Place to Hide (1970), Up the Sandbox (1972), I, the Jury (1982) and Trading Places (1983), and softcore work such as Voices of Desire (1972) and While the Cat's Away... (1972). Following her retirement from porn, she also appeared in the mainstream film Married to the Mob (1988). Willoughby began making hardcore porn in 1975. Her notable appearances include The Opening of Misty Beethoven (1976), The Farmer's Daughters (1976), Outlaw Ladies (1981), and Foxtrot (1982).

She appeared in many Pornographic magazines, most notably Penthouse and also wrote columns for such magazines as High Society and Velvet. She also arranged film release parties and toured as a Burlesque dancer. During her career in porn, Willoughby was known as an activist speaking out in favor of the porn industry.