Simpson became a British subject during World War I, shortly after graduating from Harvard and renouncing his United States citizenship.

Dorothea and Ernest Simpson had one child, Audrey C. C. Simpson (born 1924), who married firstly on 5 October 1945, American journalist Murray Rossant (died 1988, brother of architect James Rossant) and, secondly on 1 April 1949, New York advertising executive Edmund Hope Driggs III.

As his obituary in The New York Times noted, the publicity over his second wife's remarriage to the Duke of Windsor and her subsequent fame thrust him into the role of "the forgotten man".

[8] The two remained friends, however, the newspaper noted, with the now Duchess of Windsor sending him flowers when he was in hospital for surgery and Simpson offering advice and clarification when his former wife was working on her memoirs.

She was the younger daughter of Sir John Ashley Mullens, of Manor House, Haslemere, Surrey, by his wife, the former Evelyne Maude Adamson.

By this marriage Simpson had a stepdaughter, Lucinda Gaye Leveson-Gower (born 1935, married Sir Spencer Le Marchant in 1955).

He was portrayed by David Harbour in W.E., a 2011 romantic drama film about the Duke and Duchess of Windsor's courtship; the movie was co-written and directed by Madonna.