Dumas was born in 1953 in Cape Town, South Africa and grew up in Kuils River in the Western Cape, where her father had a vineyard. Dumas began painting in 1973 and showed her political concerns and reflections on her identity as a white woman of Afrikaans descent in South Africa. She studied art at the University of Cape Town from 1972 to 1975, and then at Ateliers '63 in Haarlem, which is now located in Amsterdam. She studied psychology at the University of Amsterdam in 1979 and 1980. She currently lives and works in the Netherlands and is one of the country's most prolific artists. She is also widely regarded as one of the most influential painters working today.