He was the son of Major George Smart and his wife Katherine, daughter of Sir Joseph Henry Hawley, 3rd Baronet (1813–1875), a wealthy racehorse owner, bibliophile and advocate of turf reform, and his wife Sarah (née Crosbie), who came from a landed Sussex family.
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He first began working as a trainer for Sir Joseph Hawley in 1848 at Fyfield, Wiltshire.
His father, William Harcourt Isham Mackworth (1806—1872), a younger son of Sir Digby Mackworth, the 3rd Baronet, took the additional surname Dolben after he married Frances, the heiress of Sir John English Dolben, the 4th Baronet.