[1] John Jager was born in Bistra, Austria-Hungary (now Slovenia) at Railway Guard House 365A above the village, where his father worked as a railway guard.

[2] He graduated from high school in Ljubljana in 1892, after which he studied architecture at the Vienna University of Technology under Joseph Maria Olbrich and Otto Wagner.

[1] In 1898, he toured Lower Carniola, Pivka, the Vipava region, and the Karst Plateau, where he collected folk motifs to decorate the National Café (Slovene: Narodna kavarna) in Ljubljana.

[5] In 1901, he went to Beijing at the invitation of the Austrian government to build shelters for its soldiers during the Boxer Rebellion.

[1][5] In 1918, Jager traveled to Serbia, where he worked as an inspector for a Red Cross unit in charge of rebuilding 60 villages damaged during the First World War; for this work he was made a Red Cross captain, and in 1940 he was awarded the Order of the Yugoslav Crown.