 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.