Legassick was born in Edinburgh, Scotland. In 1947 he and his parents emigrated to South Africa. He attended the Diocesan College in Cape Town. In 1960 he became a Rhodes Scholar at Balliol College, Oxford. He later completed his PhD at the University of California. He then worked at universities in the United Kingdom and Tanzania, where he became active in the ANC and the South African Congress of Trade Unions (SACTU) in exile. Together with Giovanni Arrighi, John S. Saul and others he developed an influential politico-economic analysis focusing on the contradictions engendered by the proletarianisation and dispossession of the Southern African peasantry. According to Arrighi, "Martin Legassick and Harold Wolpe...maintained that South African Apartheid was primarily because the regime had to become more repressive of the African labour force because it was fully proletarianized, and could no longer subsidize capital accumulation as it had done in the past."
