Jahr was born August 7, 1871 in Holmestrand, Norway.  At the age of 19, he immigrated to America.  He graduated from Luther College in 1896.  After Luther Jahr attended Illinois State University and from there received his Bachelor of Library Science in 1900.  In 1902, Jahr began work at the Library of Congress in Washington, DC.  For several years he was the president of the Norwegian Society in Washington.  After resigning his position at the Library in 1928, Jahr moved back to his birthplace in Norway.  He died November 6, 1931 in Norway.

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Jahr ble født i Holmestrand, 7. august 1871. Han kom til Amerika som nittenåring og studerte ved Luther College til 1896. Etter Luther studerte han ved Illinois State University der han tok bachelorgraden i bibliotekvitenskap i 1900. I 1902 begynte han å jobbe i Library of Congress i Washington, DC. I flere år var han president for Norwegian Society i Washington. Etter å ha sagt opp jobben ved biblioteket i 1928, flyttet Jahr tilbake til hjembygda i Norge. Han døde 6. november 1931.