[3] After joining the special mission to Russia for Alexander II's accession,[4] Lord Cavendish of Keighley (as he was styled at the time) entered Parliament in the 1857 general election, when he was returned for North Lancashire as a Liberal (his title "Lord Hartington", by which he became known in 1858, was a courtesy title; as he was not a peer in his own right he was eligible to sit in the Commons until he succeeded his father as Duke of Devonshire in 1891).

His lethargic manner, apart from his position as war minister, helped to associate him in their minds with a disaster which emphasized the fact that the government acted "too late"; but Gladstone and Lord Granville were no less responsible than he.

[6] Hartington became increasingly uneasy with Gladstone's Irish policies, especially after the murder of his younger brother Lord Frederick Cavendish in Phoenix Park.

After the general election of 1886 Hartington declined to become Prime Minister, preferring instead to hold the balance of power in the House of Commons and give support from the back benches to the second Conservative government of Lord Salisbury.

Early in 1887, after the resignation of Lord Randolph Churchill, Salisbury offered to step down and serve in a government under Hartington, who now declined the premiership for the third time.

Having succeeded as Duke of Devonshire in 1891 he entered the House of Lords where, in 1893, he formally moved for the rejection of the Second Home Rule Bill.

But in the autumn of 1907 his health gave way, and grave symptoms of cardiac weakness necessitated his abstaining from public effort and spending the winter abroad.

[6] He served part-time as captain in the Duke of Lancaster's Own Yeomanry from 1855 to 1873, and was honorary colonel of the 3rd (Militia) Battalion of the Derbyshire Regiment from 1871 and of the 2nd Sussex Artillery Volunteers from 1887.

He was married at Christ Church, Mayfair, on 16 August 1892, at the age of 59, to Louisa Frederica Augusta von Alten, widow of the late William Drogo Montagu, 7th Duke of Manchester.

A statue of the Duke can be found at the junction of Whitehall and Horse Guards Avenue in London, and also on the Western Lawns at Eastbourne.

[11] Historian Jonathan Parry claimed that "He inherited the whig belief in the duty of political leadership, afforced by the intellectual notions characteristic of well-educated, propertied early to mid-Victorian Liberals: a confidence that the application of free trade, rational public administration, scientific enquiry, and a patriotic defence policy would promote Britain's international greatness—in which he strongly believed—and her economic and social progress...he became a model of the dutiful aristocrat".