[2] His younger sister, Lady Mary Capel, married Alan Brodrick, 2nd Viscount Midleton.

[3] Capell was one of the founding governors of the charity, the Foundling Hospital, created in October 1739 to care for abandoned children.

He was made a Privy Councillor c. 1734, a Knight of the Garter c. 1737, and was Captain of the Yeomen of the Guard from 1739 until his death in 1743.

Before Lady Jane died in 1724, they were the parents of two children:[3] On 3 February 1726, William was married to Lady Elizabeth Russell, a daughter of the 2nd Duke of Bedford and the former Elizabeth Howland (daughter and heiress of John Howland of Streatham).

[3] Through his second daughter, he was a grandfather of two girls, Maria Eleanor Forbes who married John Villiers, 3rd Earl of Clarendon, and Katherine Elizabeth Forbes who married William Wellesley-Pole, 3rd Earl of Mornington.