 He made a fortune in banking and was said to be worth £400,000 later in life, and the richest commoner in England on his death.Duncombe was baptized on 16 November 1648, the second son of Alexander Duncombe of Drayton Beauchamp, Buckinghamshire, and his wife Mary Pawley, daughter of Richard Pawley of Whitchurch, Buckinghamshire.He was apprenticed to the London goldsmith Edward Backwell in 1665 and became in 1672 a member of the Goldsmiths' Company.