Margolyes was born in Oxford, England to Joseph Margolyes, a physician, and Ruth Walters, a real estate investor. She grew up in a Jewish family and began acting in her twenties in Cambridge.
Margolyes keeps her private life just that - private. But she has not seen any reason to keep it a secret that she is a lesbian, or as she playfully says "deliberately never been married or had children". She took some time off of her career to take care of her ill mother and spent a quarter of a million pounds on full time care for her father. She has been an Australian citizen since 2013.
Career in the theatre
Miriam also portrayed Madame Morrible in the London production of Wicked and played the minor role of Grunhilda in season three of the television show Merlin, The Changling.