Howarth was born the son of soldier Arthur Howarth and Phoebe Worsdell. He enjoyed a nearly fifty-year relationship with American LSE academic George Goetschuis and entered into a civil partnership with him in February 2006 shortly after their introduction in the UK. Goetschuis died in October 2006 at the age of 83. They lived for most of their life together in George Devine's old Thameside house in London, theatre director
Peter Gill sharing part of the property for many years. Donald Howarth is mentioned in the diary of
Joe Orton, when Orton visited Peter Gill. Donald Howarth lived between London and a countryside property in Wales, in the garden of which both he and George Goetschuis are buried. ==Theatre works==