was a British painter and draughtsman who produced history paintings and portraits. His parents and origins are unknown.
Shackleton painted several surviving portraits, for example of Henry Pelham (National Portrait Gallery), William Windham (1717 - 1761; now at Felbrigg Hall, Norfolk), and of John Bristowe, steward to the first duke of Newcastle (now in the Reitlinger Museum of Fine Art, Maidenhead).
From 1749 he was Principal Painter in Ordinary to George II and George III. He continued to be paid for portraits of the king and queen up even during 1765 - 6, when their official portraits were being done by Allan Ramsay. Several examples of his and his studio's output of royal portraits survive - one of George II dated 1755 is in the Scottish National Portrait Gallery, Edinburgh; another of George II in Room 2 of the British Museum, London (commissioned by the museum in 1759 - the Museum also holds engravings after his paintings), along with two more of George II in the Royal Collection and others in Fishmongers' Hall, London, and Maidenhead Museum.