Thomas Corbett was born on 2 June 1888 at Naini Tal in India, the son of Thomas Bartholomew Corbett, a postmaster, and his wife, Emily Harriet. He was educated at St Paul's School, Darjeeling (1897-1904), and the Royal Military College, Sandhurst (1905-7), passing out near the top of his class. On 10 January 1907 Corbett was commissioned second lieutenant and after a year with a British unit he joined Hodson's Horse, an Indian cavalry regiment. Between November 1914 and March 1918 he served on the western front, was twice promoted (captain 1915, brevet major 1917), and was awarded an MC and bar. On 28 October 1915 he married Flora Margaret (1878/9-1951), daughter of David Gold, who had previously divorced Alistair Maxwell Macdonell. In 1918-19 Corbett served as a brigade major with the Egyptian expeditionary force in Palestine before entering the Staff College, Quetta, in October 1920. In 1922 he was promoted major and held various staff appointments in southern command and at army headquarters (AHQ) India. Early in 1930 he transferred to the 2nd lancers (Gardner's Horse) and on 24 January he was appointed an instructor at the Staff College, Quetta, where he established a close friendship with Claude Auchinleck. He attended the senior officers' school, Belgaum, in 1932, before returning to his regiment, and was promoted brevet lieutenant-colonel in 1933. In September 1934 he returned to instruct at the Staff College, where he was awarded a viceroy's certificate for rescue work during the May 1935 earthquake, and was promoted colonel on 1 July 1935. On 2 August 1938 he took command of the Sialkot brigade area with the rank of temporary brigadier, and in January 1940 he was appointed brigadier, cavalry, at AHQ in New Delhi where he oversaw the conversion of Indian cavalry regiments into armoured units. Corbett was promoted acting major-general on 22 April 1940 and was instrumental in organizing and training the 1st Indian armoured division and in developing a tactical doctrine for the Indian armoured corps. He was made a CB in 1941.

In January 1942 Corbett took command of 4th Indian corps in Iraq, with the rank of acting lieutenant-general. Shortly afterwards, on 15 March 1942, he was appointed by Auchinleck (now General Sir Claude) to the key post of chief of general staff (CGS), Middle East, to the surprise of many senior British service officers in London and Cairo who knew nothing about this Indian cavalryman. As the situation in the western desert deteriorated Corbett acted as an intermediary between the commander-in-chief and his hard-pressed field commanders. When Major-General Neil Ritchie was relieved, Auchinleck briefly considered Corbett as general officer commanding Eighth Army, but then decided to take command in the field himself.

While the commander-in-chief was absent Corbett remained at general headquarters Cairo as his representative, carrying out its routine business and preparing the city and the Nile delta for defence. His inexperience of higher command, however, meant that Corbett, now clearly out of his depth, failed to impress Churchill and many senior officers who met him. In particular, his lacklustre efforts to instil a warlike spirit among the large number of troops in the rear areas exposed him to considerable criticism. Along with Auchinleck, Corbett was dismissed in the ‘Cairo purge’ in August 1942 and returned to India where he briefly commanded 7th Indian division. Corbett retired from the Indian army in 1943 and became involved in farming in Kenya. Following his first wife's death in 1951, he married in 1952 Sara (Sally) N. E. Withers (née Raymond), widow of Lieutenant-Colonel H. H. C. Withers. They had a daughter. On 28 December 1981 he died of bronchopneumonia at Panthill, Spithurst, near Barcombe, near Lewes.