Born in England, Thomas spent the majority of his early career in the Colonial Service in Africa, rising to become Governor of Nyasaland and of the Gold Coast.

In 1909, Thomas was appointed as Assistant District Commissioner, East Africa Protectorate (Nairobi, Kenya).

[1] After the war, Thomas remained as the 11th British High Commissioner in Malaya (9 November 1934 – 1 April 1946), until the Malayan Union was established and succeeded the British administration in the Straits Settlements (except for Singapore, which was created as a distinct crown colony), Federated Malay States and Unfederated Malay States, where the post of Governor-General of the Malayan Union was created.

[1] Thomas is widely associated in his role as governor with the loss of Singapore and his civilian administration's apparent failures to properly assess the growing Japanese threat and make appropriate defences.

[12] Singapore's capture by the Japanese, in conjunction with other events at the time such as the sinking of Prince of Wales and Repulse, severely undermined British prestige and contributed to the eventual end of colonialism in the region.