Treharne was educated at Cowbridge Grammar School he later studied medicine at St Bartholomew's Hospital Medical College.

His youngest son, Leslie Llewellyn Treharne died of wounds in France, during the First World War on 24 September 1915.

[6] When Richard Mullock formed the first Welsh rugby union team in 1881, Treharne was one of the youngest members contacted to join the side.

At the time he was living in Pontypridd, and is reported to have played for the club from 1880; but Treharne was still studying for Cowbridge Grammar School when the Welsh game took place.

Wales lost the match, but not as heavily as their first meeting, and the selectors only dropped two players for the next game; Cardiff's James Clare and Treharne.