In 1968, the owners of the Chicago Mustangs of the North American Soccer League (NASL) signed Kowalik. That season, he led the league in scoring with thirty goals in twenty-eight games. This led to his selection as the league MVP and a first team All Star. At the end of the season, he moved to the California Clippers. However, the NASL lost most of its teams with all west coast teams, but the Clippers, folding. The Clippers decided to leave the NASL, join a local league and exist on playing exhibition games against foreign teams. In January 1969, the team ownership decided to cease operations.
Kowalik returned to the NASL with the Chicago Sting in 1975, where he played fourteen games, scoring nine goals in 1976. He then returned to the Netherlands to play with MVV Maastricht. In 1977, he returned to the Sting where he scored only one goal in an unknown number of games. He finished his career in the 1978–1979 season in the Netherlands with MVV.