Deyna began playing youth football in 1958 with the local Włókniarz Starogard Gdański. In 1966 he made one appearance for ŁKS Łódź (on 8 October in a 0–0 draw against Górnik Zabrze). But he was quickly snapped by Legia Warsaw. In communist Poland each team had its own "sponsor". Warsaw club was much more powerful as it was a military club. Moreover it was a favourite club of the authorities. Talented player was called up into the army and in this way he had to play for Legia Warsaw. His career really got on well there, and yet in next season he was playing regularly. In 1969 and 1970 his team won the Polish Championship.
Deyna played for Poland on 97 (84 after the deduction of Olympic Football Tournament competition games) occasions, scoring 41 goals, and often captained the side. He had the ability to score from unusual positions, e.g. directly from a corner. Thanks to his skills, he was chosen several times the Football Player of the Year by Polish fans. In 1978 he captained Poland at the Football World Cup in Argentina, where the team reached the second phase.
Soon afterwards Deyna was transferred to English club Manchester City F.C., making his debut in November 1978, and being one of the first wave of overseas players to play in the English league. His time in England was marred by a series of injuries, and he left in January 1981, shortly after a change in team manager, having made only 38 appearances in all competitions. However, he was regarded as an exceptionally gifted playmaker and became a cult figure with City fans. Deyna scored thirteen goals in his time with the club. Furthermore, his seven goals in the last eight games of 1978/79 season were crucial for Manchester City avoiding relegation.
He died in a car accident in San Diego, California, aged 41. In 1994 he was chosen by the Polish Football Association (PZPN) and the readers of all Polish sports-related newspapers as the Polish Football Player of All Time. His number 10 is retired by Legia Warsaw. In June 2012 Kazimierz Deyna was buried in Warsaw's military Powazki Cemetery.