Thomas Adamson is said to have been born at Wanganui on 9 September 1845, the second son of William Adamson, a carpenter, and Mary Osborne. In his teens he followed the goldrush to Otago, but returned disappointed. A young man of impressive physique, he joined the Auckland Militia in 1865, transferring in the following year to the Wanganui unit.
Adamson was attracted to the Maori way of life, and was a keen bushman. He was befriended by Te Keepa Te Rangihiwinui (Major Kemp), leader of the pro-government Wanganui Native Contingent. Adamson often worked with Te Keepa's force, going barefoot like his comrades. In 1868 he joined the Wanganui Cavalry Volunteers.
An effective bush scout, Adamson was the type of man Colonel George Whitmore was seeking in his struggle to counter the resistance movements led by Titokowaru and Te Kooti. He was selected for Whitmore's Corps of Guides in 1869. With his brother Steve, Tom Adamson fought in the attack on Titokowaru's camp at Otautu on the Patea River on 13 March 1869. Titokowaru's followers fell back to Whakamara, and were pursued by Whitmore's forces.
The government had offered a reward of £1,000 for the capture of Titokowaru, dead or alive. Te Keepa persuaded Whitmore to offer a bounty of £10 a head for chiefs and £5 for others. His soldiers interpreted this to mean that they had to decapitate their prisoners and bring in the heads to claim payment. Adamson and another bushman, Donald Sutherland, joined in this practice.
In 1869 Tom and Steve Adamson went with Whitmore's expedition to the Urewera in search of Te Kooti, marching from Matata to Fort Galatea. After reaching Ahikereru pa, they were in the advance guard heading for Ruatahuna when it was ambushed at Manawahiwi on 7 May. Tom Adamson was shot through the right wrist. A second shot struck a revolver he carried slung across his chest, and he suffered only a cut when the bullet was deflected. The attack was repelled, and Lieutenant Gilbert Mair swiftly brought up his contingent of Te Arawa in support.
In 1870 Adamson returned to the Urewera with Te Keepa's Wanganui contingent, and took part in the attack on Waipuna pa in March. The troops, including Adamson, were ruthless in their treatment of the defeated. The principal captives were summarily executed.
In civilian life Adamson took up farming at Matahiwi on the Wanganui River. From about 1873 he lived with Nika Waiata. In 1883 they went to Ngaurukehu, where they cleared land and started farming. They had no children of their own but raised several, and adopted a boy and a girl. Having moved back to Wanganui, Tom Adamson and Nika Waiata were married there on 22 November 1910. Nika Adamson died on 16 May 1911 at the age of 69.
Thomas Adamson possessed bravery and skill as a bush fighter at a time when these qualities were much sought after by the government armed forces. He demonstrated a streak of cruelty, however, in the treatment of helpless prisoners. He was awarded the New Zealand Cross in 1876 'for good and gallant services as a scout and guide throughout the campaign of 1868–69.' He died at Wanganui on 29 December 1913.