Thomas Hickman was born in France – according to his own account at Ponto de Mare – on 13 January 1848, the son of Thomas Edward Hickman, an ironworker, and his wife, Ellen Bond. In 1850 or 1851 his parents returned to their home at Bilston, Staffordshire, England. Nothing else is known of Thomas junior's early life.
Hickman arrived at Wellington, New Zealand, on the Wild Duck on 20 December 1867, and obtained employment as a junior clerk for the engineering firm of E. W. Mills and Company. In 1868 he volunteered for the Wellington Rangers as a bugler. He was a member of a relief party which went to Turuturumokai redoubt in south Taranaki after it had been attacked by the forces of Riwha Titokowaru on 12 July 1868, and was involved in the first attack on Te Ngutu-o-te-manu.
The Rangers were disbanded in October 1868, and on 9 October Hickman joined the New Zealand Armed Constabulary. He served initially at Patea and Kai Iwi, and in 1869 went to the East Coast, where he was a member of a detachment ambushed by Te Kooti Arikirangi. He later saw service at Tauranga and Taupo. By 1874 he was back in Taranaki serving at Pukearuhe, New Plymouth and Okato. He was required to carry telegrams between Okato and Opunake: there was no telegraph line in the area, perhaps because of fears that supporters of Te Whiti-o-Rongomai would pull it down. These duties ended when a line was installed in 1876.
Thomas Hickman married Martha Selina Golding at New Plymouth on 31 October 1876. In the same year he was promoted to mounted sergeant in charge at Urenui; he frequently carried the mail to the Waitara Post Office, as well as carrying out general troopers' work between New Plymouth and Pukearuhe. In 1880 and 1881 he several times visited the Parihaka and Okato districts to investigate clashes between Maori and settlers, and was present at Parihaka in 1881 when John Bryce and a contingent of Armed Constabulary arrested Te Whiti and Tohu Kakahi.
After September 1884 John Ballance, native minister and minister of defence, introduced the 'one-policeman' policy at Parihaka as part of a generally more conciliatory attitude towards the Maori. Many of the forces in north Taranaki were withdrawn; Hickman, appointed to liaise with Te Whiti, Tohu and their followers, became the Armed Constabulary's race relations expert in the area. In spite of continuing problems over land he gained the confidence of both Maori and Pakeha and built a reputation for fairness and justice. In late 1884 he was mounted orderly to Sir George Whitmore in Wellington. He was transferred to Opunake in 1885.
Hickman was appointed to the New Zealand Police Force when it was established on 1 September 1886. He was employed particularly in intelligence work among the Maori living between Opunake and Hawera. From February 1887 to March 1888 he watched the movements of Titokowaru and other 'troublesome Natives'. He was moved to Pungarehu in 1888 and for the next decade was involved in all the policing activity there. In 1897 he arrested a man named Enoka, who was subsequently hanged for the murder of his wife. Hickman's commanding officer, Inspector Francis McGovern, recommended him for promotion and praised the ability, determination and tact with which he had discharged his duties. The commissioner of police paid him a £5 reward instead, and in November 1898 he was transferred to Opunake as mounted constable in charge. He also served as clerk of the Magistrate's Court, and government appointee on the Taranaki District Maori Council, as well as filling several minor posts. His ability to speak Maori was an asset to him in these enlarged duties. Described as a good Maori linguist, he was known as 'Mr Tommy' by the Maori people of his district. He was also known as 'the smallest and smartest' policeman in New Zealand – he was just five feet five inches in height.
Thomas Hickman retired on 30 June 1911 on a pension of £155 12s. a year. He lived in Wanganui for many years, where he was active in the Maori war veterans' association, then moved back to New Plymouth. He compiled a number of journals about his life. On his 82nd birthday he was described as 'a very remarkable man, full of tact and sound common sense'. He died at New Plymouth on 4 September 1930 and was buried at Te Henui cemetery. Hickman was survived by his wife, six sons and two daughters.