Emilius was bred by Colonel John R Udney, a notable turfite from Aberdeen who won the Oaks and the One Thousand Guineas Stakes with Corinne (br f 1815 Waxy) and was the breeder of Truffle (b c 1808 Sorcerer). He was purchased by Thomas Thornhill for 1800 guineas following his retirement from the turf in 1824. Mr Thornhill was a popular breeder and owner from Riddlesworth, Norfolk, who had success with the Derby winners Sam (ch c 1815 Scud) and Sailor (ch c 1817 Scud), the Oaks winner Shoveler (b f 1816 Scud) and the One Thousand Guineas winner Extempore (b f 1840 Emilius).

"The Druid" said of Emilius that he was "Orville's best son, inherited his plain head, but was not so coarse in appearance. He was a muscular, compact horse, with a great chest and powerful arms, but rather short legs, the hinder ones being unusually straight. He had a somewhat large middle piece, good back ribs, and a very muscular muscular neck, slightly arched, but not too long."

Although he didn't start as a two year old his three year old season was brilliant. Undefeated, he won the 1823 Derby Stakes, the Riddlesworth and the Grand Duke Michael Stakes. His four year old season was less successful and he retired to the stud. There he got the Derby winners Priam* (b c 1827), who was also a Champion Sire, and Plenipotentiary (ch c 1831). He also sired the Prix du Jockey Club (French Derby) winner Gambetti (b c 1845). His other classics winners included the Oaks winner Oxygen (b f 1828), the St Leger winner Mango* (br c 1834), the Two Thousand Guineas winner Riddlesworth* (ch c 1828) and the One Thousand Guineas winners Barcarolle (b f 1835), Extempore (b f 1840) and Preserve (ch f 1832). Emilius was Champion Sire in 1830 and 1831.

Following the death of Mr Thornhill in 1844 his stud was broken up and Emilius, still in good health, was leased to Mr Jacques at Easby Abbey in Yorkshire where he died on August 25, 1847, apparently from a dinner of unbruised oats which he was quite unable to chew. He is said to have been buried in a paddock close to the White Cannons of Easby and a stone which was used as the threshold to his box was built into the wall as a memorial.