[5] He joined the British Army on 19 October 1838 as an assistant surgeon to the 17th (Leicestershire) Regiment of Foot[6][7] and was stationed in India with the 14th (The King's) Regiment of Light Dragoons until 1847.

Upon his return to England, he was appointed surgeon to the 58th (Rutlandshire) Regiment of Foot[9] and sent to New Zealand.

In New Zealand he wrote extensively about disease statistics among Māori and European populations and climatology.

[5] Thomson's book, The Story of New Zealand: Past and Present, Savage and Civilized (1859), is generally considered to be the first scholarly history of the island country.

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