De Matthaeis spent the majority of his fifteen-year professional career with the New York Cosmos, with brief periods in Italy. He was invited to a National teamtraining camp towards the end of his career. During his time with the Cosmos, he had the privilege of playing alongside legends such as Carlos Alberto, Franz Beckenbauer, and Giorgio Chinaglia. He appeared 38 times over 4 years for the New York Cosmos from 1981 to 1984. During his career with the Cosmos he scored 17 times and assisted on 12 goals in both indoor and outdoor competitions.