Deyna was born in Starogard Gdański to Franciszek (1911–1976) – a dairy worker, and Jadwiga (1917–1981) – a housewife. He had six sisters and two brothers – Henryk and Franciszek, who also were footballers; Henryk played for Włókniarz Starogard Gdański, while Franciszek was a Starogardzki KS player.
Deyna began playing youth football in 1958 at his local club 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 up by Legia Warsaw. In communist Poland each team had its own "sponsor". The Warsaw club was much more powerful as it was the military club. Moreover it was the favourite club of the authorities. Deyna was called up into the army and in this way he had to play for Legia Warsaw. He made a name for himself during his first season, becoming one of Legia's most important players. In 1969 and 1970 his team won the Polish Championship. After his performances at the 1974 World Cup, European top-teams like AS Saint-Etienne, AC Milan, Inter Milan, AS Monaco, Real Madrid and Bayern Munich tried to acquire his services but he was unable to join due to the communist regime in Poland, preventing him to move to Western-Europe. Real Madrid was so determined to buy Deyna, that they sent a shirt to Warsaw with his name and number "14".
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, for example directly from a corner. Because of his achievements and talents, he was chosen Football Player of the Year several times 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 Manchester City made a managerial change, having made only 43 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 the 1978/79 season were crucial for Manchester City in their relegation battle.
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 and the Sockers. In June 2012 Kazimierz Deyna was buried in Warsaw's Powązki Military Cemetery.