This is the first biography of Lord Headley, who made international headlines in 1913 when he defied convention by publicly converting to Islam. Drawing on previously unpublished archival sources, this book focuses on Headley's religious beliefs, conversion to Islam, and work as a Muslim leader during and after the First World War. Lord Headley slipped into obscurity following his death in 1935, but there is growing recognition globally that he is a pivotal figure in the history of Western Islam and Muslim-Christian relations; this book evaluates the strengths and weaknesses, successes and failures of the man and his work, and considers his significance for contemporary understandings of Islam in the Global West.