Harley's considerable collection of coins and medals – 520 lots in all – was auctioned by Christopher Cock at his house in the Great Piazza, Covent Garden over six days, from 18 March 1742.

Through his wife, he inherited Welbeck Abbey in Nottinghamshire, and Wimpole Hall in Cambridgeshire.

He also acquired a considerable amount of land in the West End of London which was developed during his life.

On 31 August 1713 he married Lady Henrietta Cavendish Holles (1694–1755), only daughter and heir of the 1st Duke of Newcastle and his wife, the former Lady Margaret Cavendish, daughter of the 2nd Duke of Newcastle-upon-Tyne.

They had two children: Lord Oxford and Mortimer died in London in 1741 and was buried in the vault of the Duke of Newcastle in Westminster Abbey.