Martyn was a Silver medallist for Team GB at the 1968 Olympic Games in Mexico City and is still considered to be one of the greatest athletes to come out of Wales. Martyn became a swimming coach whilst he was still a competitor himself and his dedication to producing the greatest swimming team has continued. Martyn was for a period the Scottish National Coach and has more recently returned home to his home of Wales to be the National Performance Director for the Welsh Swimming team. In 2010 Martyn led the Welsh team to their 2nd highest medal tally ever at a Commonwealth Games, a remarkable achievement given the testing conditions experienced by the team. Martyn is also technical selector for British Swimming; he has contributed to selecting and building highly successful junior and senior teams.