In 1789 the spelling was updated to Wellesley-Pole, just as other members of the family had changed Wesley to Wellesley.

He was born as William Wesley, at Dangan Castle, the second son of Garret Wesley, 1st Earl of Mornington, by his marriage to Annie Hill, a daughter of Arthur Hill-Trevor, 1st Viscount Dungannon.

[1] Due to the debts of their father, the Wesley family entered into financial stringency.

This was partially alleviated following the death in 1781 of the childless William Pole, of Ballyfin in Ireland, his godfather and the husband of his great-aunt Ann Colley, who bequeathed his estates to Wesley, on the condition which was usual in such situations that he should adopt the surname "Pole".

[2] A Tory, Mornington was a Member of the Irish Parliament for Trim from 1783 to 1790, and of the British House of Commons for East Looe from 1790 to 1795, and Queen's County from 1801 to 1821.