Fenwick played rugby as a schoolboy for Caerphilly Grammar Technical School, and later joined Taffs Well RFC. In 1971 he switched to Beddau where he played 51 games before moving on the first class team Bridgend. While at Bridgend Fenwick earned all of his 30 international caps for Wales, making his début against France in 1975, in which he scored a try after only five minutes, and finished the game with a personal tally of 9 points. He was a member of the 1977 British Lions tour to New Zealand, in which he played in all four Tests. He also captained Wales in the Centenary game against the All Blacks in 1980. He also played for a World XV on 9 August 1980 against in Buenos Aires, losing 36-22.