Kralj was born in Zagorica near Dobrepolje in Lower Carniola in 1900. He studied sculpture in Prague between 1920 and 1923 and then at Vienna, Paris and Venice. Some of his best-known works are the wall paintings in various churches. His 1954 illustrations for Fran Levstik's Martin Krpan are also some of the most recognizable images of the story. He won the Levstik Award in 1950 for his illustrations for Pravljica o carjeviču Jeruslanu (The Story of Prince Jeruslan). He died in Ljubljana in 1975.
In 1972 he received the Prešeren Award for his life's work.