Sedbury was a chestnut racehorse bred in 1734 by Andrew Wilkinson of Boroughbridge, Yorkshire, by Mr Croft's Old Partner out of Lord D'Arcy's famous Old Montague Mare. Mr Wilkinson sold Sedbury when a yearling to Mr Mann of the same town. He first ran in 1738, winning twenty guineas at Hambleton, beating Lord Halifax's No Name amongst others. Mr Mann then sold him at the end of 1739 to a Mr Martindale of St. James's Street, London, and the jockey in the present work is shown wearing his colours. The jockey is likely to be James Larkin, who went on to become a leading horse trainer of his day, and was father of John Larkin, painted by Stubbs in 1768 on Otho (Tate Britain).
Between 1738 and 1744 Sedbury won nineteen of his twenty-four races, of which his two best were for King's Plates at Newmarket in 1740 and 1741. Both times he beat a particularly fine racehorse called Elephant. In 1744, Sedbury's last season, he won 60 guineas at Newmarket, defeating the Duke of Ancaster's Brisk. He retired to stud at Leeming Lane in Yorkshire and died circa 1759.
Sedbury is described in Picks Register, volume I, as 'a horse of exquisite beauty, of great justness of shape and form, and was indisputably the best horse of his size at the time of his running'.
Seymour is known to have painted Sedbury several times. There are various engravings of the present composition, published by the likes of James Roberts, Thomas Butler, Thomas Spencer and T. Bradford among others as one of several popular series of racehorses in the 1740s-1750s. Judy Egerton has expressed reservations regarding the attribution of this picture to Seymour.