A cobbler's son, Deakin began work at age 13 in a South Wales steel plant, becoming an active trade unionist during World War I and a full-time union official in 1919. In 1932 he was appointed national secretary of the General Workers' group of the Transport and General Workers' Union, the largest in Britain. In 1940 he succeeded Ernest Bevin in the general secretaryship when Bevin joined the War Cabinet. From 1945 until his death Deakin's dominance of British trade unionism was perhaps even greater than Bevin's had been in the 1930s.

Deakin was an astute wage negotiator, but he supported the Labour government's policy of wage restraint. A firm anticommunist, in 1948 he led the noncommunist unions out of the World Federation of Trade Unions (of which he was president) and helped form a rival organization, the International Confederation of Free Trade Unions. After the fall of the Labour government in 1951, his opposition to Aneurin Bevan's left-wing group within the Labour Party helped to maintain both Clement Attlee's party leadership and the adherence of Labour to a pro-U.S. policy. In 1954 Deakin was made a privy councillor.