William Pole-Tylney-Long-Wellesley, 4th Earl of Mornington was an Anglo-Irish nobleman notorious for his dissipated life of living.

Earl of Mornington is a title in the Peerage of Ireland. It was created in 1760 for the Anglo-Irish politician and composer Garret Wellesley, 2nd Baron Mornington. On the death of the fifth earl in 1863, it passed to the Duke of Wellington; since that date, the title has generally been used by courtesy for the heir apparent to the heir apparent to the dukedom.