Treharne was educated at Cowbridge Grammar School he later studied medicine at St Bartholomew's Hospital Medical College. He became a Doctor of Medicine, and was prominent in local politics. He was the president of Cadoxton Junior Conservative Club, was a member of both the Barry Lodge of Freemasons and Barry District Council. He was married to Margaret Louise (Crooke), and they had at least two children. In fact, when he married Margaret Louise in 1899, he was a widower and both his sons were from his first wife, Lydia Elizabeth Billings. His youngest son, Leslie Llewellyn Treharne died of wounds in France, during the First World War on 24 September 1915.