Hardinge entered the army in 1799 and, during the Napoleonic Wars, served with distinction as a staff officer in the Peninsular War (1808–14). In the Hundred Days (1815), he was a brigadier general with the Prussian army at the Battle of Ligny and had his wounded left arm amputated two days before the Battle of Waterloo. In 1820–44 he was a member of Parliament, serving as secretary of war twice and as chief secretary for Ireland twice.

In 1844 he succeeded his brother-in-law, Lord Ellenborough, as governor-general of India. There he encouraged education by offering government employment to college-educated locals and sought to suppress human sacrifice. He also discouraged suttee and infanticide. He began construction of the Ganges canal and developed plans for an Indian railway system. He served in the First Sikh War and by the Treaty of Lahore (March 1846) sought to establish a friendly, if much-reduced, Sikh state. For his part in the war, Hardinge was awarded a viscountcy (May 1846).

In 1852 Hardinge succeeded the Duke of Wellington as commander in chief of the British army. Though responsible for the establishment of the first training camp at Chobham, for the purchase of the Aldershot military training camp, and for the introduction of the improved Enfield rifle, his lax administration and unwise choice of commanders were partly responsible for the disasters suffered by the British in the Crimean War (1853–56). Hardinge was nevertheless promoted to field marshal in 1855.