In 1820 Hardinge was returned to parliament as member for Durham. On 4 April 1823 he was appointed Clerk of the Ordnance and on 9 June 1828 he accepted the office of Secretary at War in Wellington's ministry. Returned as Member of Parliament for St Germans in 1830, for Newport in 1831 and for Launceston in 1832, he served as Chief Secretary for Ireland in 1830 and 1834 to 1835. He was Secretary at War once again in Sir Robert Peel's cabinet from 1841 to 1844. He was promoted to lieutenant-general in 1841. In May 1844 he succeeded Lord Ellenborough as Governor-General of India. He was advanced to Knight Grand Cross of the Order of the Bath on 1 July 1844. Following the death of Maharajah Ranjit Singh tribal war broke out and the first Sikh War ensued in 1845. Hardinge, waiving his right to the supreme command, offered to serve as second in command under Sir Hugh Gough. At the Battle of Mudki on 18 December 1845 Gough commanded the right flank and Hardinge commanded the left flank. After further British successes at the Battle of Sobraon on 10 February 1846, the Battle of Ferozeshah on 21 December 1845 and the Battle of Aliwal on 28 January 1846, Hardinge concluded the campaign with the Treaty of Lahore with Maharajah Duleep Singh on 9 March 1846 and the Treaty of Amritsar with Maharajah Gulab Singh on 16 March 1846. He was created Viscount Hardinge of Lahore and of King's Newton in Derbyshire on 7 April 1846. Recognising an annuity of £5000 being paid by the East India Company, Parliament provided that Viscount Hardinge should continue to receive his full salary as Governor General. Under a subsequent Act, in recognition of his "great and brilliant services", Parliament settled an annuity of £3000 on Lord Hardinge and the next two heirs male of his body, although this was not to be paid if the East India Company paid an annuity.
Commander-in-Chief
Hardinge returned to England in 1848, and became Master-General of the Ordnance on 5 March 1852; he succeeded the Duke of Wellington as commander-in-chief of the British Army on 28 September 1852. While in this position he had responsibility for the direction of the Crimean War, which he endeavoured to conduct on Wellington's principles — a system not altogether suited to the changed mode of warfare. He was promoted to brevet general on 20 June 1854 and field marshal on 2 October 1855. A commission was set up to investigate the failings of the British military during the Crimean campaign. As Hardinge was delivering the report of the commission to Queen Victoria and Prince Albert, he collapsed with a stroke. Albert helped him to a sofa, where despite being paralysed on one side, he continued to deliver his report, apologizing for the interruption. He was also colonel of the 97th Regiment of Foot from 4 March 1833 and of the 57th Regiment of Foot from 31 May 1843. Hardinge resigned his office of commander-in-chief in July 1856, owing to failing health, and died on 24 September 1856 at South Park near Tunbridge Wells. There is a memorial to him at St John the Baptist, Penshurst. He is buried in the churchyard at St. Peter, Fordcombe.