The first WM missionary to Africa. While stationed in Cornwall, he was recruited by Thomas Coke and in 1811 sent to Sierra Leone in response to an appeal from some of the liberated slaves who had settled there. He sailed with three schoolmasters from the Dewsbury Circuit, Jonathan Raynar, John Healey and Thomas Hirst. After a 52-day voyage they were welcomed and helped by the Governor and the Anglican chaplain. Warren worked effectively for eight months before succumbing to the climate of what became known as the 'White Man's Grave'. Raynar returned to England and entered the ministry, serving in Antigua; his companions continued in Freetown until the arrival of William Davies in 1815.