Horace Thomas was born on 28 July 1890 in Bridgend, Wales. He was educated at Bridgend County School, where he started playing rugby. In 1906, he won a scholarship to study at Monmouth School. There, he played rugby, but also cricket, hockey, and did gymnastics and athletics. 

Endowed with an exceptional voice, in 1909 he won a prestigious scholarship to study choral singing at King's College, Cambridge. 
 
He left for England in 1909 and continued to play rugby during his three years at Cambridge, mainly for Blackheath, but also for London Welsh. 
 
He played in the Barbarians tour matches in Newport and Leicester and twice for Swansea. He played in the Varsity Match against Oxford in December 1912 and caught the eye of the Welsh selectors. A few days later, he was selected to play for Wales against South Africa in 1912, and against England in 1913, in the opening match of the Five Nations Championship.  
 
He retired from rugby in 1913 and moved to India, where he took up a management position with a shipping company. There he kept playing rugby and cricket, and joined the Calcutta Football Club, where he became captain. 
 
At the outbreak of the First World War, he returned to Wales to serve his country. He was promoted to Second Lieutenant in the 16th Battalion, The Rifle Brigade and sent to France in March 1916. He took part in the fighting during the offensive on the Somme and found himself at the heart of violent action during the Battle of Hamel. He was killed on 3rd September. 

He has no known grave and is commemorated on the Thiepval Memorial (Pier and Face 16B and 16C).