Born 29 July 1861, the second son and youngest of the five children of Evan Thomas, a farmer, of Cefn Banal in the parish of Llanbadarn Odwyn, Cardiganshire, and Catherine Richards, his wife. David Thomas received an elementary education at the Ystradmeurig Grammar School, and from the age of twelve helped his father on the farm. He sustained a fracture of the lower third of the femur about the age of 14 which prevented him from following the plough any longer. He learnt English and entered the London Hospital in 1879, where he acted as house physician and was appointed house surgeon to Sir Frederick Treves in 1884. He acted for a time as senior clinical assistant at St Luke's Hospital, and migrated to New South Wales in 1888. After spending short periods at Summer Hill and Kogarah, he settled at Manly, where he practised until his death. Here he became a leader of surgical thought, established the Manly District Hospital in 1896, and became its senior medical officer. He was president of the New South Wales branch of the British Medical Association in 1914, and one of the foundation Fellows of the Australasian College of Surgeons in 1927. At the beginning of the war in 1914 Thomas was appointed medical officer to the troopship Moravian; in 1915 he was granted a commission as lieutenant in the RAMC, and was employed for the next sixteen months on the Scottish Travelling Board. He then returned to New South Wales, and acted as the repatriation medical officer. He married Charlotte Herhington on 21 November 1888, who survived him with two sons, both members of the medical profession. He died at Elsmere, Manly, Sydney, New South Wales on 15 February 1931. Thomas was as active in municipal as in medical matters, was an alderman of the suburb, 1905-08, and was the Government medical officer in Manly. Throughout his active medical career he was a firm believer in the principle of Friendly Society Lodge practice.