He enjoyed a near 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 2007 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 now splits his time between London and a countryside property in Wales, in the garden of which George Goetschuis is buried.

Donald Howarth, Plays One: Sugar in the Morning, A Lily in Little India, All Good Children, Three Months Gone, Oberon Modern Playwrights, 2000