Sancho was born on a slave ship bound for Colombia. His mother died soon after arriving and his father killed himself soon afterwards rather than live as a slave. When he was two he was brought to England by his owner and given to his three maiden sisters in Greenwich.
He taught himself to read and write, entered the service of the Dukes of Montague and became a trusted servant and favorite "pet" of English high society. He also sat for his portrait by Gainsborough.
He was left a legacy in the 1750s, but wasted it on women and gambling and later opened a grocery shop, married and had six children. He was a friend of the author Laurence Sterne.
His Letters, published after his death, were very popular. In 1997 the National Portrait Gallery in London held an exhibition on his life.