Casimir inherited a kingdom weakened by war and made it prosperous and wealthy. He reformed the Polish army and doubled the size of the kingdom. He reformed the judicial system and introduced a legal code, gaining the title "the Polish Justinian". Casimir built extensively and founded the Jagiellonian University (back then simply called the University of Krakow), the oldest Polish university and one of the oldest in the world. He also confirmed privileges and protections previously granted to Jews and encouraged them to settle in Poland in great numbers.

Casimir left no sons. When he died in 1370 from an injury received while hunting, his nephew, King Louis I of Hungary, succeeded him as king of Poland in personal union with Hungary.