Sisulu was born in 1912 in Ngcobo in the Union of South Africa, part of what is now the Eastern Cape province (then the Transkei). Not unusual for his generation in South Africa, he was not certain of his birthday, but celebrated it on 18 May. His mother, Alice Mase Sisulu, was a Xhosa domestic worker and his father, Albert Victor Dickinson, was a white civil servant and magistrate. Dickinson did not play a part in his son's upbringing: Sisulu reportedly met him only once, in the 1940s, before he died in the 1970s. Sisulu and his sister, Rosabella, were raised by his mother's family, who were descended from the Thembu clan. He was close with his uncle, Dyantyi Hlakula, who was passionate about Xhosa culture and who oversaw his initiation. Although he was technically of mixed race, Sisulu identified strongly as black and as Xhosa.
