He was appointed governor to the prince of Wales, afterward George III, in 1751; and after the accession of the latter to the throne, in 1761, he was appointed as special ambassador to Mecklenburg-Strelitz, to negotiate a marriage between King George and Charlotte of Mecklenburg-Strelitz (Princess Charlotte), whom he conducted to England.

His proposal to impose a tax of 10% on the rents of absentee landlords had to be abandoned owing to opposition in England; but he succeeded in conciliating the leaders of Opposition in Ireland, and he persuaded Henry Flood to accept office in the government.

[2] He died there shortly afterwards by accidentally drowning in a well while trying to rescue his favourite dog, which had fallen into the well while the pair had been out for a walk.

[1] He succeeded in rescuing the dog despite losing his life; several hours after going missing, Harcourt was found head first down the well with only his lower legs and feet visible above the water and the dog sitting on his feet.

[3] He married, on 16 October 1735, Rebecca Samborne Le Bass (died 16 January 1765), daughter and heiress of Charles Samborne Le Bass, of Pipewell Abbey, Northamptonshire.