Kassam was born in Hammersmith Hospital, Acton, London...
He is the son of Tanzanian immigrant parents of Gujarati origin from Hillingdon, Greater London.
He was raised in the Ismaili sect of Shia Islam, but wrote in 2016 that he had not been a practising Muslim for over a decade.
Kassam is an atheist, stating that Christopher Hitchens' rejection of religious faith ("religions are versions of the same untruth") inspired him.
Kassam was educated at Bishopshalt School, a state comprehensive school in Uxbridge and St Helen's College, Hillingdon, and then studied Politics at the University of Westminster.