While still a student, he wrote his first book, Aus dem Weichseldelta (From the Vistula Delta), which introduced readers to the unique landscape between Gdansk and Elblag. His own authentic reports of experiences and adventures always played an important role: In 1878, a year before his move from Insterburg to Königsberg, he published Aus baltischen Landen, a remarkable treatise on the Curonian Spit and its inhabitants. Passarge now traveled more and more frequently through other European regions and countries, for example to Sweden, Portugal and Spain, in order to write further reports on his experiences. 

In 1903, one of Passarge's most important cultural-historical works, Ein ostpreußisches Jugendleben, was published. Despite his strong ties to his homeland, Ludwig Passarge always remained a traveler, recording his impressions in books and articles. He once described himself as a "right vagabond" who led "an enviable and delicious life". In addition to East Prussia and Scandinavia, Passarge also traveled to the Balkans in the 1890s to collect materials for the first Baedeker travel guides. Passarge continued his travels into old age. In 1912 he died during a trip to the Odenwald. 

Passarge's part in communicating Scandinavian literature within the German-speaking world should not be forgotten. During his stays in Scandinavia, he became acquainted with the works of Henrik Ibsen and Bjørnstjerne Bjørnson, whose works he translated into German. Another of Passarge's lasting achievements was the new translation of the complete works of the Prussian-Lithuanian priest Kristijonas Donelaitis, which appeared in Halle in 1894 and contained, in addition to animal fables, the verse poem Metai (Die Jahreszeiten). Johannes Bobrowski based his novel Litauische Claviere (1966) on this translation by Ludwig Passarge.