Ponsonby's wife was Lady Henrietta-Frances Spencer. She was Lord Granville's lover prior to his marriage to Lady Harriet Cavendish, the Countess of Bessborough's niece. Lord Granville fathered two illegitimate children through her: Harriette Stewart and George Stewart.
Bessborough Gardens in London SW1 is named after him.[1]
John Ponsonby married Lady Maria Fane, daughter of John Fane, 10th Earl of Westmorland and Sarah Anne Child, on 16 November 1805 at Berkeley Square, London. The Henrietta Frances Spencer listed above was in fact his mother, wife of Frederick Ponsonby. Source: thePeerage.com
References
^The London Encyclopaedia, Ben Weinreb & Christopher Hibbert, Macmillan, 1995, ISBN 0-333-57688-8