He was commissioned Colonel of the 1st Battalion of the Lancashire Supplementary Militia on 1 March 1797; this regiment subsequently became the 2nd Regiment, Royal Lancashire Militia. He was breveted as a colonel in the regular Army with seniority from that date, retaining the rank until his regiment was disembodied, which occurred at the end of 1799. He resigned his commission as colonel on 13 April 1847.
Naturalist
In 1834 he succeeded his father as 13th Earl of Derby and withdrew from politics, instead concentrating on his natural history collection at Knowsley Hall, near Liverpool. He had a large collection of living animals: at his death there were 1,272 birds and 345 mammals at Knowsley, shipped to England by explorers such as Joseph Burke. From 1828 to 1833 he was President of the Linnean Society. Many of Derby's collections are now housed in Liverpool's . Several species were named after him, for example the Derbyan parakeet, Psittacula derbiana and an Australian species of parrot named firstly by Nicholas Vigors as Platycercus stanleyii, in 1830 when he was Lord Stanley, and referred to in the vernacular as "The Earl of Derby’s Parrakeet" by the author John Gould in the sixth volume of his magnum opusBirds of Australia. However the latter species was found to heve been named previously as Platycercus icterotis, and thus Platycercus stanleyii was found to have been an invalid name due to the pre-existence of a published description for the species, according to "the inviolable laws of precedence in deliberations on biological nomenclature".
Marriage and issue
On 30 June 1798 he married his first cousin Charlotte Margaret Hornby, second daughter of Rev. Geoffrey Hornby, of Scale Hall, near Lancaster in Lancashire, High Sheriff of Lancashire in 1774 and a Deputy Lieutenant of Lancashire, Colonel of a regiment of Lancashire militia, by his wife Lucy Smith-Stanley a daughter of James Smith-Stanley, Lord Strange, and a sister of Edward Smith-Stanley, 12th Earl of Derby. Charlotte's brother was Edmund Hornby of Dalton Hall, near Burton, Westmorland, a Member of Parliament for Preston, Lancashire, from 1812–1826, who married his first cousin Lady Charlotte Stanley, a daughter of Edward Smith-Stanley, 12th Earl of Derby. By Charlotte Hornby he had issue: