Born in London; his parentage is uncertain, and there was a report that he was a natural son of George III. He entered the Royal Academy, London schools in 1775 and exhibited portraits at the Royal Academy, London 1780-1809, being elected Associate Royal Academician, London 1793 and Royal Academician 1795.
He was called Portrait Painter to the Prince of Wales in the Royal Academy, London catalogue of 1789, by which time he was living very near Carlton House. After the death of Reynolds, to whom Hoppner owed much, he contested the lead in English portraiture with Lawrence. In 1802 he visited Paris. He died in London on 23 January 1810 in his fifty-first year.