Thomas began preaching at age 15. He graduated from New College, London University in 1863 and was ordained, was married, and sailed to China the same year. Within three months, his wife, Caroline (Godfrey), died. A year later he resigned from the LMS, feeling that unreached fields should be given priority over already occupied Shanghai, but he soon asked to return and to be sent to Mongolia.
While he was waiting for reinstatement, a chance meeting with two Korean traders, secret Catholics, led him to negotiate a trip to that forbidden country to distribute Bibles for the National Bible Society of Scotland. He spent two and a half months there in 1865 and learned some of the language. Against advice he returned to Korea in 1866 as interpreter on an armed American trading ship, arriving at a time when uninvited foreign trade was still forbidden and a raging persecution of secret Catholic believers was resulting in the execution of thousands. The ship was attacked near Pyongyang and no one survived. Thomas was reportedly beheaded giving a Bible to his executioner. He was the first Protestant martyr in Korea.