His father was born illegitimately and named William Melville at birth but had his name legally changed to William Milbank by Royal Warrant in 1792 in order for him inherit the estates of his father, Mark Milbank.[5] His maternal grandfather was John Wise of Woolston, Devon.[2]
He succeeded to his father's estates in North Yorkshire in 1802, including Barningham Park, which had been home to the Milbank family since 1690. He was elected to Brooks's in February 1818.[2] His wife, Lady Augusta, created the pinetum on the family estate.[3]
Milbank was returned for the borough of Camelford in 1818 on his father-in-law Lord Darlington's interest, but was unseated on petition. Two years later in 1820, however, he was returned unopposed. At the general election in summer 1822, Milbank and Sheldon Cradock, another nominee of his father-in-law, were returned for Camelford ahead of two candidates of The Earl of Yarmouth (who had to leave the House upon becoming the 3rd Marquess of Hertford).[2]
^L. G. Pine, editor, Burke's Genealogical and Heraldic History of the Peerage, Baronetage and Knightage, 99th edition (London: Burke's Peerage Ltd, 1949), page 1382.
^"No. 19462". The London Gazette. 31 January 1837. p. 232.