 He died aged at 91 on June 3, 1855 in Danzig (Gdańsk).In honor of Mrongovius, the name of the former East Prussian city Sensburg ("Żądzbork" in Polish) was changed to Mrągowo in 1947 by the new Polish administration.Mrongovius was known for preserving and teaching Polish cultural heritage and language in Danzig to people from the territories affected by the Partitions of the Polish-Lithuanian Commonwealth.