Johannes Weidemann entered the Kaiserliche Marine in august 1914 in the first Matrosendivision. In february 1915 he was on UB9 and UB1, in march 1915 on UB7 in Konstantinopel. January 1916 he joined the third Uboat-flotilla and after becoming Leutnant der Reserve on may 25, 1917, he joined the fifth Uboat-flotilla in september 1917. In april 1918 he became watch officer on U90, and he left the Reichsmarine on january 21, 1920