Born the son of the Reverend Henry Hardinge, Rector of Stanhope, and Frances Hardinge (née Best) and educated at Durham School, Hardinge entered the British Army on 23 July 1799 as an ensign in the Queen's Rangers, a corps then stationed in Upper Canada. He was promoted to lieutenant by purchase in the 4th Regiment of Foot on 27 March 1802 and transferred to the 1st Regiment of Foot on 11 July 1803 before becoming a captain of a company by purchase in the 57th Regiment of Foot on 21 April 1804. In February 1806 he was sent to the newly formed Staff College at High Wycombe.
He saw action at the Battle of Roliça on 17 August 1808, at the Battle of Vimeiro on 21 August 1808, where he was wounded, and at Corunna on 16 January 1809 where he was by the side of Sir John Moore when the latter was killed. He was promoted to major on 13 April 1809 and appointed deputy-quartermaster-general in the Portuguese army and was present at many of the battles of the Peninsular War. Promoted to lieutenant-colonel in 1811, he saved the day for the British at Battle of Albuera on 16 May 1811 by taking responsibility at a critical moment and strongly urging General Cole's division to advance. He took part in the Battle of Vitoria on 21 June 1813, where he was wounded again, and was also present at the Battle of the Pyrenees in July 1813 and the Battle of Nivelle on 10 November 1813. He commanded the Portuguese brigade at the Battle of Orthez on 27 February 1814 and the Battle of Toulouse on 10 April 1814. He was appointed Knight Commander of the Order of the Bath in January 1815.
When war broke out again in 1815 after Napoleon's escape from Elba, Hardinge returned to active service as a brigadier. Attached to the staff of the allied Prussian Army under Marshal Blucher, he was present at the Battle of Ligny on 16 June 1815, where he lost his left hand by a shot, and thus was not present at Waterloo two days later. Wellington presented him with a sword that had belonged to Napoleon. Hardinge remained attached to the Prussian army of occupation in France until 1818.
He was promoted to brevet colonel on 19 July 1821 and to major-general on 22 July 1830.
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.
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. The church is part of a united benefice with Penshurst.
In 1821 he married Lady Emily Jane, seventh daughter of Robert Stewart, 1st Marquess of Londonderry. His elder son, Charles Stewart (1822–1894), who had been his private secretary in India, was the 2nd Viscount Hardinge. The younger son of the 2nd Viscount, Charles Hardinge (b. 1858), became a prominent diplomat, and was appointed Viceroy of India in 1910, being created Baron Hardinge of Penshurst.