Smith began his career in the army as an ensign in 1805 and served with distinction in South America (1807) and, during the Napoleonic Wars, in Spain (1808–14). In the War of 1812 Smith was with the British forces that captured and burned Washington, D.C. He returned to Europe in time to be active in the Waterloo campaign.

After military duties in England and Jamaica, he was transferred to Cape Colony (1828) and participated in the Cape Frontier War (Cape Frontier Wars) of 1834–35, during which he made a historic ride from Cape Town to Grahamstown, covering 600 miles in six days to allay the fears of colonists exposed to attack. With the rank of general, Smith was sent to India (1840), where he fought the Sikhs in 1845 and 1846. He won renown at the Battle of Aliwal (Jan. 28, 1846) by leading the final charge in person.

Smith was created a baronet and promoted to major general, and he was sent back to the Cape in December 1847 as governor and high commissioner. When he annexed the territory called the Orange River Sovereignty, the Boers' leader, Andries Pretorius, raised a Boer force. With characteristic vigour, Smith attacked and defeated the Boers at Boomplaats (Aug. 29, 1848). Dissatisfaction with Smith's precipitate manner and with his handling of the costly Cape Frontier War of 1850–53 resulted in his being recalled in March 1852 to Britain, where he held various military posts until his death.