 Born in Christchurch on 10 February 1858, Millton was the eldest son of William Newton Millton, a sea captain and runholder, and his wife Caroline Millton (née Stockman).He was educated at Christ's College from 1869 to 1876, and went on to become a barrister and solicitor.On 23 April 1885, Millton married Elizabeth Anderson at St Paul's Church in Christchurch.