[2] Although he began working as a model builder, he soon followed his father into engineering, completing an apprenticeship with Ross & Duncan before studying at the Royal Technical College.
He then worked as a draughtsman designing boilers and joined the new Association of Engineering and Shipbuilding Draughtsmen (AESD).
He was elected to its executive in 1917, where was a close associate of general secretary Peter Doig, who had studied with Thomson at the School of Art, and he became the union's convenor later the same year.
[2] During World War I, Thomson was joint editor of The Guildsman, the Glasgow publication of the National Guilds League.
[2] In 1935, Thomson was elected to the General Council of the Trades Union Congress (TUC), and he served as President of the TUC in 1946/47 He was also active in the National Federation of Professional Workers and the Labour Party, where he was secretary of the Science Advisory Committee.