Bred by John Borlase Warren (1677-1747) of Stapleford, near Nottingham, he was described as remarkable for his great size and strength. Careless was a full-brother to Fearnought* (b c 1755), a notable stallion in America, and a half-brother to Warren's Sportsman (b c 1753 Cade), who sired the dam of Pot8os (ch c 1773 Eclipse). Following the death of Mr Warren, Careless was purchased by Evelyn Pierrepont (1711-1773), the 2nd Duke of Kingston.

Careless may be expressing both sabino and rabicano genes, as in his portrait he appears to have white hairs throughout his face and shoulder, which can be indicative of sabino colouring, and possibly a spray of white in his tail, with more white hairs behind the saddle, suggesting the presence of rabicano. Although his dam, Silvertail, is called a bay in the General Stud Book, in some racing calendars her colour is noted as roan. She has two daughters recorded as roan in the stud book, who may have been rabicanos as well.

Racing from 1755 through 1761, he won seven King's Plates, as well as a Great Subscription purse, in which he defeated the previously unbeaten Atlas. The latter race generated much interest since Atlas had beaten Careless twice before, and a great deal of money rested on the outcome.

He retired from the turf to the Duke's stud at Holme Pierrepont, near Nottingham, where his fee was 10 guineas. In the stud he was noteworthy for his contribution to American bloodstock, siring Mousetrap* (b c 1771), and Sister to Croney (f 1768c), the dam of Obscurity* (ch c 1777 Eclipse), both stallions of enduring value.