Mansfield is best known for his judgment in Somersett's Case on the legality of keeping slaves in England. The English had been involved in the slave trade since 1553, and by 1768, ships registered in Liverpool, Bristol and London carried more than half the slaves shipped in the world. James Somersett was a slave owned by Charles Stewart, an American customs officer who sailed to Britain for business, landing on 10 November 1769. A few days later Somersett attempted to escape. He was recaptured in November and imprisoned on the ship "Ann and Mary", owned by Captain John Knowles and bound for the British colony of Jamaica. Stewart intended to sell him there. However, three people claiming to be Somersett's godparents, John Marlow, Thomas Walkin and Elizabeth Cade, made an application before the Court of King's Bench for a writ of "habeas corpus", and Captain Knowles was ordered to produce Somersett before the Court of King's Bench, which would determine whether his imprisonment was legal.