‘Delme’ Thomas was educated at St Clears School, West Wales . His representative rugby began at Youth Level and he played for Carmarthen Athletic and Llanelli. Thomas gained his first cap in 1966 against Australia and went on to play a total of 25 matches.

On his third successive Lions selection he toured Australia and New Zealand and won three Test caps. Although a hard man and a versatile player Thomas had great skills, lineout expertise and a pleasant demeanor off the field which earned him many plaudits from admiring spectators and opponents alike. He captained the Llanelli Scarlets in the 1972/3 season when the club defeated the All Blacks back in Wales, and as recognition for this achievement he was awarded with the BEM. He has toured with the British Lions a total of 3 times, gaining 7 caps.