Slovo was born in Obeliai, Lithuania, to a Jewish family that emigrated to the Union of South Africa when he was eight. His father worked as a truck driver in Johannesburg. Although his family were religious, he became an atheist who retained respect for "the positive aspects of Jewish culture". Slovo was educated at King Edward VII School and left school in 1941 and found work as a dispatch clerk. He joined the National Union of Distributive Workers and, as a shop steward, was involved in organising a strike.