He was the only son of Lt.-Gen. George Forbes, 4th Earl of Granard and the former Letitia Davys (d. 1778), who were first cousins (their mother's were sisters).
[2] His paternal uncle was Admiral of the Fleet John Forbes, the father of his cousins, Katherine Wellesley-Pole, Countess of Mornington (wife of William Wellesley-Pole, 3rd Earl of Mornington) and Maria Villiers, Countess of Clarendon (wife of John Villiers, 3rd Earl of Clarendon).
Dorothea's brother, Henry Bayly, later took the surname of their maternal grandfather was elevated to the peerage as the Earl of Uxbridge.
Before Dorothea's death on 19 February 1764, they were the parents of one child:[4] Forbes remarried on 22 April 1766 to Lady Georgiana Augusta Berkeley (1749–1820) in London.
Together, they were the parents of six children, including:[4] Upon his death on 15 April 1780, he was succeeded in his title by his eldest son from his first marriage, George Forbes.