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Born in Scotland on 15 June 1792, he was son of John Mitchell of Carron Works and was brought up from childhood by his uncle, Thomas Livingstone of Parkhall, Stirlingshire. On the death of his uncle, he joined the British army in Portugal as a volunteer, at the age of sixteen. On 24 June 1811, at the age of nineteen, he received his first commission as 2nd Lieutenant in the 1st Battalion 95th Rifles (later the Rifle Brigade / Royal Green Jackets). Utilising his skills as a draughtsman of outstanding ability, he was occasionally employed in the Quartermaster-General's department under Sir George Murray.

In 1831 a runaway convict named George Clarke, who had lived with Aborigines in the area for several years, claimed that a large river called Kindur flowed north-west from the Liverpool ranges in New South Wales to the sea. Charles Sturt believed that the Murray-Darling system formed the main river system of New South Wales and Mitchell wanted to prove Sturt wrong. Mitchell formed an expedition consisting of himself, assistant surveyor George Boyle White and 15 convicts who were promised remission for good conduct. Mitchell took 20 bullocks, three heavy drays, three light carts and nine horses to carry supplies, and set out on 24 November 1831 to investigate the claim. On reaching Wollombi in the Hunter Valley, the local assistant surveyor, Heneage Finch, expressed a desire to join the expedition. He had established his credentials by surveying a route from Sydney to Wollombi, so Mitchell approved his request, provided he obtained extra supplies and men, and he followed along later.  The expedition continued northward, climbed the Liverpool Range on 5 December, and made Quirindi on 8 December. Shortly afterwards Finch arrived but inexplicably had not brought provisions, so Mitchell immediately sent him back to get them. By 11 December the expedition had reached Wallamoul Station near Tamworth, the northern extent of white settlement at the time.  Mitchell continued his northward push into new territory, taking local Kamilaroi people to assist in finding water and to express his peaceful intentions. By early January 1832 he was in the vicinity of the present town of Narrabri, reached the Gwydir River in mid-January, and found the Barwon by the end of the month, where he set up camp. Mitchell left the main party and made a reconnaissance down the Barwon until he found its junction with the Gwydir. After Mitchell returned to the main camp, Finch arrived with tragic news. He had been travelling with three convicts and provisions but, because of a shortage of water, the group had separated. He left two men behind while he attempted to locate the main expedition. Having failed to do that, on the second day he returned to the campsite to discover that it had been ransacked. Supplies were scattered and beneath the wreckage were two dead men. The immediate effect was that Mitchell decided to abandon the expedition and return south. The party reached the Gwydir on 8 February and was near the site of the attack on 17 February. Mitchell buried the two bodies and salvaged some equipment. Once back at Wallamoul, Mitchell placed White in charge of the main party, while he returned hastily to Sydney. He was satisfied that there was no truth about the river Kindur claimed by Clarke. Fourteen years after the attack, Mitchell revealed that the convicts had indulged in sexual relations with Aboriginal women.
Thomas Mitchell (explorer)