Born 10 September 1861 at Pensarn, Llandyssul, Cardigan, the eldest child of Evan Thomas, farmer, and Mary Lewis, his wife. He was educated at a small private school at Llandyssul under the headmastership of Gwilym Marlais, and afterwards entered the London Hospital Medical School. He won the Hutchinson prize essay in 1890 for a dissertation on fractures of the skull, and later in life presented the manuscript to the library of the Welsh National School of Medicine. At the London Hospital he served as house surgeon, and returning to Cardiff he acted as house surgeon at the Cardiff Royal Infirmary 1888-92, and then was appointed assistant surgeon to the Dinorwic Quarries Hospital. In 1895 he was elected assistant surgeon to the Cardiff Royal Infirmary, becoming surgeon in 1901, and consulting surgeon on his resignation of the post in 1921. During the South African war he served with the Welsh Hospital in 1900, and after the death of his seniors took charge of it, was decorated CB, and was awarded the medal with three clasps. He then returned to Cardiff, interesting himself more especially in the treatment of fractures and dislocations, although he undertook general operative surgery. Being already a major in the RAMC Territorial Force, he was gazetted temporary lieutenant-colonel on 29 April 1916, and was promoted colonel, AMS on 29 September 1917, Western Command. As deputy inspector of orthopaedics he helped Robert Jones to organize that side of the Army Medical Service during the years 1914-18. He was instrumental in founding the Prince of Wales Hospital, Cardiff, for limbless sailors and soldiers. The hospital afterwards became the orthopaedic centre for Wales. He also acted as consulting surgeon to King Edward VII Welsh National Memorial Hospital, to the Hamadryad Hospital, Cardiff, to the Cardiff Provident Dispensary, to Porth Hospital, and to the Bridgend Cottage Hospital. He was also interested after the war in the Rookwood Hospital at Llandaff and the War Memorial Hospital, Cardigan. A man of great ability as an organizer, he took an active part in establishing the Welsh National School of Medicine and in the affairs of the National University of Wales, of which he was the junior deputy chancellor. He was also a member of council of the University College of South Wales and Monmouthshire, and a councillor of University College, Aberystwyth. He retired from private practice in July 1914, but continued his public work and was a member of the Council of the Royal College of Surgeons 1918-33, and president of the orthopaedic section at the Cardiff meeting of the British Medical Association in 1928. He lived latterly at Llwyndyrys, Llechryd, Cardiganshire, and there interested himself in the archaeology of the district. He was a magistrate for Glamorgan and Cardigan, and a deputy lieutenant and high sheriff (1907-08) for the latter. Sir John Lynn Thomas is described as being childlike in disposition, friendly by nature, and transparently interested in his own affairs. Somewhat combative, he involved himself in various litigations relating to his professional services, and always fought to the end. The Southern v Lynn Thomas and Skyrme case went against him in 1906, and a public subscription from the medical profession defrayed the costs (*Lancet*, 1908, 1, 879). Above middle height and massively built, he was distinguished by his large head upon which the hair grew in untamed profusion. Welsh was clearly his native language, for he always spoke English in a soft voice and with a distinctive accent. He married in 1892 Mary Rosina, only daughter of Edward Jenkins of Cardiff. She helped him greatly in establishing Welsh Hospitals during the Boer war in 1900 and the war of 1914-18. She was found drowned near her home in the River Teist on 12 February 1938. Sir John Lynn Thomas died on 21 September 1939, leaving two daughters. His remains were cremated at Pontypridd. He left the greater part of his estate upon trust to purchase the house Llwyndyrys and adjoining lands, and when acquired to maintain it and the stones and discoveries thereon, with a view to handing it over to the Welsh nation or to such other body as would be prepared to take it; otherwise he left his property to the Prince of Wales Hospital, Cardiff.