William Pole-Tylney-Long-Wellesley, 4th Earl of Mornington (22 June 1788 – 1 July 1857) was an Anglo-Irish nobleman notorious for his dissipated style of living.

[2] He was born William Wesley-Pole on 22 June 1788 in London and baptised at St George's, Hanover Square, on 19 July.

His first marriage was on 14 March 1812 to Catherine Tylney-Long, known in fashionable London society as "The Wiltshire Heiress",[4] and believed to be the richest commoner in England.

During this period he enjoyed a political career, first as a Tory Member of Parliament for the pocket borough of St Ives from 1812 until 1818, and then for Wiltshire, where his wife's family was influential.

On one occasion in 1814, Long-Wellesley held a grand fête in Wanstead House and its gardens to celebrate his uncle the Duke of Wellington's victory over Napoleon, attended by the Prince Regent, a number of other members of the royal family, and over a thousand dignitaries.

As a notorious rake he was unable to afford to make prudent investments in his first wife's property, but nor could he sell or mortgage it, having only a life interest in it (enjoying its income).

As life tenant, he arranged with other beneficiaries for the demolition and carving up of Wanstead House's estate, the proceeds of which repaid some of his great debts.

His uncle, the Duke, fighting one of his furious defensive actions, intervened on behalf of the children to keep the hapless William from his father.