 Mackay made her stage debut at the American Shakespeare Festival, Stratford, Connecticut, in 1970.She appeared in many productions at The Cleveland Play House from 1975–1978, including "Man and Superman", "Dark at the Top of the Stairs", "Relatively Speaking", "Of Mice and Men", "Great Expectations", and "Little Foxes".She played Alice in "You Can't Take It with You" at Center Stage, Baltimore, Maryland, in 1979, "Crimes of the Heart" at the Ahmanson Theatre, Los Angeles (1983), and "The Dining Room", Plaza Theatre, Dallas, Texas (1983).