Martyn played as a goalkeeper from 1987 until 2006. Martyn moved to Crystal Palace in 1989 where he became the first million- pound goalkeeper in British Football. He remained at Palace for seven seasons and made 349 appearances for the club. In 1996, Martyn signed for Leeds United, setting another record fee for a goalkeeper at £2.25 million. Martyn was part of the squad that eliminated FC Barcelona, S.S. Lazio and Deportivo La Coruna in the 1999-2000 UEFA Champions League campaign. Martyn was voted Leeds greatest ever goalkeeper.