He was an officer in World War I. After 1918 he was a police officer. In 1935 he was transferred to the Wehrmacht In 1937 he was promoted to colonel.[1]
Jahr committed suicide on 21 January 1943 when his unit was being overrun as part of the encirclement during the Battle of Stalingrad.
Fellgiebel, Walther-Peer (2000) [1986]. Die Träger des Ritterkreuzes des Eisernen Kreuzes 1939–1945 — Die Inhaber der höchsten Auszeichnung des Zweiten Weltkrieges aller Wehrmachtteile [The Bearers of the Knight's Cross of the Iron Cross 1939–1945 — The Owners of the Highest Award of the Second World War of all Wehrmacht Branches] (in German). Friedberg, Germany: Podzun-Pallas. ISBN978-3-7909-0284-6.