 Julio Mariner was an exceptionally good-looking bay horse with no white markings bred in Wiltshire by the Fonthill Stud which was managed by James Ian Morrison.He was sired by Blakeney the winner of the 1969 Epsom Derby, who was standing at the National Stud.Julio Mariner's dam Set Free was a moderate racehorse, with a best Timeform rating of 90, but she became an outstanding broodmare.