Macdonald was born in Glasgow, the daughter of Janet Rewat (née Williams) and Joseph Henry Hosgood MacDonald. She has Scottish and Welsh ancestry. Macdonald was educated at the University of Edinburgh, from which she graduated in 1972. She credits fellow Scot Ian Charleson with supporting and encouraging her to follow her theatrical aspirations, and she later contributed a chapter to the 1990 book, For Ian Charleson: A Tribute. Macdonald moved to London after university, and worked as an actress with the 7:84 Theatre Company and at the Royal Court Theatre. Although her acting career included 7 years of television work, she eventually left it, due in large part to stage fright.