After leaving school at the age of 14,[2] he began working in the office of the Amalgamated Society of Dyers, Finishers and Kindred Trades.
[7] In the late 1930s, during the Spanish Civil War, Tewson was active in the Aid Spain Movement, serving as Vice-Chairman of the Basque Children's Committee, an offshoot of the National Joint Committee for Spanish Relief, founded to care for the nearly 4,000 Basque children evacuated to the UK on the ship Habana in May 1937.
[11] Tewson succeeded Walter Citrine as General Secretary of the TUC in 1946,[1] supporting the post-war economic recovery of Europe, and assisting in creating a trades union advisory Committee for the Marshall Plan.
In 1949 he was the secretary of the conference at Geneva during which the International Confederation of Free Trade Unions (ICFTU) was created, and served as its President from 1951 to 1953.
[12] Tewson retired as General Secretary in 1960, and in November of that year was appointed a part-time member of the London Electricity Board.