 He was sworn of the Irish Privy Council on 15 September 1758.Gardiner inherited the estates of his mother's family, the Stewarts, following the death of William Stewart, 3rd Viscount Mountjoy and 1st Earl of Blessington, on 14 August 1769.He died later the same year and was succeeded by his eldest son Luke, for whom the Mountjoy title was revived in 1785.