Doncaster was foaled at the Sledmere Stud, Yorkshire, Great Britain and was sired by "The Emperor of Stallions", Stockwell, who had won both the 2,000 Guineas and the St. Leger Stakes; Stockwell was a leading sire during his later years, producing many classic winners.
Doncaster, a chestnut with a white blaze originally named "All Heart and No Peel", was raised at the Sledmere Stud before he was sent to the Tattersalls auction.