Bred by Mr Plummer from the redoubtable North Yorkshire mare Alice Hawthorn (b f 1838) he was a half brother to Queen's Vase winner Oulston (b c 1852 Melbourne) and to Sweet Hawthorn (br f 1858 Sweetmeat) the taproot mare of Family 4-k. There is little doubt that Thormanby was sired by Windhound as Melbourne was a pure bay and could not have sired a chestnut. He was purchased by trainer Mathew Dawson (1829-1898) for James Merry (1805-1877) as a yearling for £350. Merry apparently thought so little of him that he made Dawson feed him for several weeks at his own expense. Merry bred the Oaks and St Leger winner Marie Stuart (ch f 1870 Scottish Chief) and the St Leger winner Sunbeam (b f 1855 Chanticleer) and owned such horses as Chanticleer (gr c 1843 Birdcatcher), the Derby winner Doncaster (ch c 1870 Stockwell) and Saunterer (bl c 1854 Birdcatcher).

Thormanby was described as a pale chestnut who darkened with age, his near hind leg was white to above his ankle and his face displayed a "peculiar" blaze. He was thought to be generally plain looking and lacking substance. The Sporting and Dramatic News said of him "His shoulders were not very grand, and an uphill course suited him best. For this reason Goodwood did not suit him, but his level, strongly-knit back, and powerfully-arched loins, sent him up the Ascot incline like a steam engine. His quarters were drooping, a peculiarity inherited by most of his stock, his arms and thighs were fairly muscular, and his cannon-bones short. He had not much bone, and was slightly inclined to be leggy. In fact he was a type of one of those wiry, light-fleshed, greyhound style of horses, whose abilities so often belie their looks".

Despite Mr Merry's opinions he turned into a very fine racehorse and won the Derby by two lengths from the Two Thousand Guineas winner The Wizard (br c 1857 West Australian) and the Ascot Gold Cup the following year by the same margin. He retired to stud in Yorkshire at the Croft Stud Farm, near Darlington, where he covered alongside Saunterer (bl c 1854 Birdcatcher). His initial crops were thought heavy and slow although he eventually produced many fine runners. He later moved to the Park Paddocks at Newmarket and then back north to the Moorlands Stud Farm near York where Scottish Chief (b c 1861 Lord of the Isles) was in residence. His best son was probably Atlantic (ch c 1871) who attained some success in France. Thormanby also got the Two Thousand Guineas winner Charibert (ch c 1876) and the One Thousand Guineas winner Hester (b f 1867). He was Champion Sire in 1869.

Thormanby was said to have "dropped dead" in March of 1875.