Martyn migrated to Australia as a child with his family settling in Adelaide. He played 192 games as a goalkeeper with West Adelaide in the former national soccer league from 1977-1986 and in his first season in the NSL, he was voted goalkeeper of the year and was a constant presence when West Adelaide won the 1978 NSL championship, conceding only 27 goals in 26 matches.
He made 13 appearances for the Socceroos from 1979-1983.
He was appointed Head Coach of the South Australian Sports Institute men's football program in 1994, was assistant coach with the Qantas Joeys from 2001 and named Head Coach of the Qantas Joeys in 2007. He was in an inductee of the South Australian Football Hall of Fame.
Martyn was Head Coach of the Qantas Joeys for 18 matches (W11-D3-L4). Under his stewardship, the Qantas Joeys became the first Australian team to win an ASEAN title when they won the ASEAN Youth Championships in Jakarta in 2008.
Martyn dedicated his life to football with a particular focus on developing youth players. His experience and personality meant that he was universally respected and liked by all within the football community and his results admired by his peers.