Sturt sailed with some prejudice against the colony but found the conditions and climate so much better than he expected that his feelings completely changed, and he developed a great interest in the country. Governor of New South Wales Sir Ralph Darling formed a high opinion of him and appointed him major of brigade and military secretary. Sturt became friendly with John Oxley, Allan Cunningham, Hamilton Hume and other explorers. He was keen to explore the Australian interior, especially its rivers.
In early 1828 Governor Darling sent Sturt and Hume to explore the area of the Macquarie River in western New South Wales. It was not, however, until 10 November that the party started. It consisted of Sturt, his servant, Joseph Harris, two soldiers and eight convicts and on 27 November he was joined by Hamilton Hume as his first assistant. Hume's experience and resourcefulness proved very useful to his leader. A week was spent at Wellington Valley breaking in the oxen and horses, and on 7 December the real start into comparatively little known country was made. It was a drought year and the greatest difficulty was found in getting sufficient water. The party returned to Wellington Valley on 21 April 1829. The courses of the Macquarie, Bogan and Castlereagh rivers had been followed, and though its importance was scarcely sufficiently realized, the Darling River had been discovered. This expedition proved that northern New South Wales was not an inland sea, but deepened the mystery of where the western-flowing rivers of New South Wales went.
In 1829 Governor Darling approved an expedition to solve this mystery. Sturt proposed to travel down the Murrumbidgee River, whose upper reaches had been seen by the Hume and Hovell expedition. In place of Hume, who was unable to join the party, George MacLeay went "as a companion rather than as an assistant". A whaleboat built in sections was carried with them which was put together, and on 7 January 1830 the eventful voyage down the Murrumbidgee was begun. In January 1830 Sturt's party reached the confluence of the Murrumbidgee and a much larger river, which Sturt named the Murray River. It was in fact the same river which Hume and Hovell had crossed further upstream and named the Hume. Several times the party was in danger from the aborigines but Sturt always succeeded in propitiating them.
Sturt then proceeded down the Murray, until he reached the river's confluence with the Darling. Sturt had now proved that all the western-flowing rivers eventually flowed into the Murray. In February 1830, the party reached a large lake which Sturt called Lake Alexandrina. A few days later, they reached the sea. There they made the disappointing discovery that the mouth of the Murray was a maze of lagoons and sandbars, impassable to shipping.
The party then faced the ordeal of rowing back up the Murray and Murrumbidgee, against the current, in the heat of an Australian summer. Their supplies ran out and when they reached the site of Narrandera in April they were unable to go any further. Sturt sent two men overland in search of supplies and they returned in time to save the party from starvation, but Sturt went blind for some months and never fully recovered his health. By the time they arrived back in Sydney they had rowed and sailed nearly 2,900 kilometres of the river system.
Sturt briefly served as Commander on Norfolk Island where mutiny was brewing among the convicts, but in 1832 he was obliged to go to England on sick leave and arrived there almost completely blind. In 1833 he published his Two Expeditions into the Interior of Southern Australia during the years 1828, 1829, 1830 and 1831, of which a second edition appeared in 1834. For the first time the public in England realized how great was Sturt's work, for Governor Darling's somewhat tardy but appreciative dispatch of 14 April 1831, and his request for Sturt's promotion, had had no result, and nothing came of the request by Sir Richard Bourke who had succeeded Darling that Viscount Goderich should give "this deserving officer your Lordship's protection and support". Though it seems to have been impossible to persuade the colonial office of the value of Sturt's work his book had one important effect. It was read by Edward Gibbon Wakefield, and led to the choice of South Australia for the new settlement then in contemplation. In May 1834, in view of his services, Sturt applied for a grant of land intending to settle on it in Australia, and in July instructions were given that he was to receive a grant of , Sturt on his part agreeing to give up his pension rights. In September he was married to Charlotte Green and almost immediately sailed for Australia.
Sturt wanted to settle the debate as to whether there was an inland sea. In August 1844 Sturt and a party of 15 men, 200 sheep, six drays and a boat set out to explore north-western New South Wales and to advance into central Australia. Travelling along the Murray and Darling rivers before venturing to the Great Dividing Range they passed the site of Broken Hill, but were then stranded for months by the extreme summer conditions near the present site of Milparinka. When the rains eventually came Sturt pressed on into central Australia until they discovered the Simpson Desert, at which point they were unable to go further and turned back to Adelaide.
Sturt later undertook a second expedition to reach the centre of Australia, but he contracted scurvy in the extreme conditions and his health broke down. He was forced to abandon the attempt. John Harris Browne, surgeon on the expedition, assisted Sturt, took over leadership of the party and after travelling brought it back to safety.
On 30 December 1851 he resigned his position and was given a pension of £600 a year and settled down on of land close to Adelaide and the sea. But the gold discoveries had increased the cost of living, and in March 1853 Sturt and his family sailed for England. He lived at Cheltenham and devoted himself to the education of his children.
In 1856 he applied for the position of Governor of Victoria. However, his age, uncertain health, and comparatively small income were against him. In 1859 the settlers at Moreton Bay requested that Sturt might be appointed the first Governor of Queensland, but again a younger man was chosen. By 1860 Sturt's three sons were all in the army, and the remainder of his family went to live at Dinan to economize after the expenses of education and fitting out. Unfortunately the town was unhealthy and in 1863 a return was made to Cheltenham. In 1864 Sturt suffered a great grief in the death of one of his sons in India. In March 1869 he attended the inaugural dinner of the Colonial Society, at which Lord Granville mentioned that it was the intention of the government to extend the Order of St Michael and St George to the colonies. Sturt allowed himself to be persuaded by his friends to apply for this distinction, but afterwards regretted he had done so when he heard there were innumerable applications.
His health had been very variable and on 16 June 1869 he died suddenly. He was survived by his widow, two sons, Colonel Napier George Sturt, R.E. and Major-general Charles Sheppey Sturt, and a daughter. Mrs Sturt was granted a civil list pension of £80 a year, and the same title as if her husband's nomination to the order of St Michael and St George had been gazetted. Reproductions of portraits by Crossland and Koberwein will be found in Mrs N. G. Sturt's Life, which suggest the charm and refinement of Sturt's character.
He is commemorated by the City of Charles Sturt and suburb of Sturt in Adelaide, the electoral Division of Sturt in Adelaide's eastern suburbs, Charles Sturt University in regional New South Wales, and the Sturt Highway from Wagga Wagga to Adelaide as well as the Sturt's desert pea, the Sturt's Desert Rose and Sturt's Stony Desert.
His home, known as "The Grange", in the Adelaide suburb of Grange is preserved as a museum.
The Australian actor Rod Taylor, whose middle name is Sturt, is his great-great grand-nephew.