Tewson succeeded Walter Citrine as General Secretary of the TUC in 1946, 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.
Tewson took over as General Secretary of the TUC from Walter Citrine in 1946, playing a key role in Europe's post-war economic recovery and helping establish a trade union advisory committee for the Marshall Plan. He was also instrumental in creating the International Confederation of Free Trade Unions (ICFTU) at a conference in Geneva in 1949, serving as its president from 1951 to 1953.