Born in 1415, he graduated at the university of Cracow and in 1431 entered the service of Bishop Zbygniew Olesnicki (1389-1455), the statesman and diplomatist.
In 1440, on returning from Hungary, whither his master had escorted King Wladislaus II., Dlugosz saved the life of Olesnicki from robbers.
Dlugosz brought Olesnicki the red hat from Rome in 1449, and shortly afterwards was despatched to Hungary to mediate between Hunyadi and the Bohemian condottiere Giszkra, a difficult mission which he most successfully accomplished.
But though he thus sacrificed his own prospects to the cardinal's good pleasure, Dlugosz was far too sagacious to approve of the provocative attitude of Olesnicki, and frequently and fearlessly remonstrated with him on his conduct.
In his account, however, of the quarrel between Casimir and Olesnicki concerning the question of priority between the cardinal and the primate of Poland he warmly embraced the cause of the former, and even pronounced Casimir worthy of dethronement.
When Olesnicki died in 1455 he left Dlugosz his principal executor.