After completing his Abitur in Wismar in 1896, Neckel studied German philology at Munich (1896–1897), Leipzig (1897–1898) and Berlin (1898–1902), where he earned his doctorate in 1900 under Andreas Heusler.

From 1935 to 1937 he was founding Head of the Old Norse Division of the Department of Germanic Studies at the University of Göttingen, then from 1937 to 1940 Professor of Germanic Philology at Berlin, where he was, however, unable to work due to illness;[2] he had a "nervous condition" from which he had barely recovered when he died suddenly of a pulmonary infection.

[6] Neckel resisted the politicisation of his department at Berlin[7] and was open-minded on race and its relevance to his discipline;[8] nevertheless, the increasingly völkisch point of view in his writings, his initial support for Kummer and Herman Wirth, and his advocacy of the autochthonous theory of the origin of the runes have led some to see a marked decline in the calibre of his scholarly work beginning in the mid-1920s.

[9] His former teacher Heusler wrote repeatedly to his friend Wilhelm Ranisch that he seemed "no longer entirely sane" and that he seemed to have developed "an unhealthy ambition, not to say megalomania".

[11] Heusler and others have considered conflict between ideologues within the Nazi regime, specifically between the Amt Rosenberg, the Ministry and the Ahnenerbe, at least partly to blame for his banishment to Göttingen.