His mother was Jane, third daughter of Sir John Dalrymple, 4th Baronet, and sister of the seventh and eighth Earls of Stair; she died in 1833.

[3] As a moderate liberal he unsuccessfully contested Cockermouth in 1835, but was successful at the following election on 15 February 1836, and continued to represent the constituency till 1 July 1852.

Defeated at the general election of that date, he was returned unopposed on 28 June 1853 for Stroud, and sat for that town till 11 November 1868.

He criticised severely, and at times with personal bitterness, the ecclesiastical policy of Lord John Russell's ministry of 1847, as being far too favourable to the bishops.

On 26 April 1850, in the discussion on the Ecclesiastical Commission Bill, Horsman smartly attacked the bishops, and roused Goulburn to denounce him as "a disappointed man" foiled of his hopes of office.

In March 1855, when Lord Palmerston became prime minister and the Peelites withdrew from the cabinet, Horsman was made Chief Secretary for Ireland, and was sworn a member of both the British and Irish Privy Councils.

With Robert Lowe, afterwards Viscount Sherbrooke, he resisted the Reform Bill brought in by William Ewart Gladstone in March 1866.