Isaacs was born in Finsbury to a Methodist family. He married Flora Beasley (1884–1962), daughter of Richard William Beasley and Mary Ann Brett, in 1905. He worked as a printer and became active in trade union organising early in life, becoming General Secretary of the National Society of Operative Printers and Assistants (NATSOPA) from 1909. This post, which he held for forty years, also took him onto the General Council of the Trades Union Congress. He was also active in the Labour Party.
In the 1924 election Isaacs lost his seat, but when in 1927 the sitting Labour MP for Southwark North resigned after leaving the party, he was the natural choice to be the new candidate. However Isaacs failed to take the seat in the byelection and had to wait until the 1929 general election to return to Parliament. Thomas, now Secretary of State for Dominion Affairs, reappointed him as Parliamentary Private Secretary.
When Thomas joined Ramsay MacDonald in the National Government, Isaacs remained with the Labour Party, and in consequence again lost his seat in the 1931 general election when the Labour Party was heavily defeated. He failed by 79 votes to regain his seat in 1935. Concentrating on union affairs through the 1930s, Isaacs was appointed to a Royal Commission on Workmen's Compensation in 1938. In 1945 he was President of the World Trade Union Conference.
After the MP for Southwark North died in 1939, Isaacs was finally able to regain the seat. When Labour formed the government after the 1945 election, he was appointed Minister of Labour and National Service. In 1945, he also attended the World Trade Union Conference in London alongside many renowned trade unionists. Part of his responsibility was to oversee the successful demobilisation of the wartime British Armed Forces. Following the arrival at London of the troopship Windrush in 1948, carrying about 500 Jamaican men said to be seeking work, Isaacs stated to Members of Parliament:
"I don't know who sent these men ... it is bound to result in difficulties ... we can give no assurances that they can be found suitable work. I hope no encouragement will be given to others to follow them."[1]
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