A centre forward during his playing days, Mariner began his career with Chorley. He became a professional player in 1973 with Plymouth Argyle, where he scored 61 goals in 155 appearances and is considered to be one of the club's best players. He joined Ipswich Town in 1976, where he achieved domestic and European success under the guidance of Bobby Robson. He was called up to play for the England national team during his time at Portman Road, and went on to represent his country at the 1980 European Championships and the 1982 World Cup. He spent two years with Arsenal and then Portsmouth before finishing his career abroad. He played for clubs in Australia, the US, and Malta.