Subsequently, authoring a number of important works on a variety of subjects, de Vries was in 1926 appointed Chair of Ancient Germanic Linguistics and Philology at the University of Leiden.

Combined with his university duties, de Vries was a leading member of the Maatschappij der Nederlandse Letterkunde and the Royal Academy of Dutch Language and Literature, led several civil organizations, edited a number of encyclopedias and magazines, and was instrumental in establishing folklore studies as a scientific discipline.

When democracy was restored in the Netherlands in 1945, he was imprisoned for several years, fired from his university, expelled from the learned societies in which had previously been a leading member, and deprived of the right to vote.

[3] After having graduated from the Hogere Burgerschool, de Vries studied Dutch, German, Sanskrit and Pali under Jan te Winkel at the University of Amsterdam from 1909 to 1913, where he received his BA and MA with great distinction.

[5] De Vries recounted his experiences in the war in the novel Een singeling in the mass (1918), which was published under the pseudonym Jan van Lokeren.

[3] Students of de Vries would later describe him as a brilliant teacher, and his ability in this regard is substantiated by the large number of influential doctoral thesises that were completed under his supervision.

[4] He was a gifted and highly productive writer, with an unusual ability to analyze and present difficult problems in a clear manner.

[3] He was the editor and driving force behind the fifth edition of Winkler Prins's Algemene Encyclopedie (General Encyclopedia), which was published in 16 volumes in 1932–1938.

As such, de Vries was requested to write the volume on Germanic religion for Hermann Paul's Grundriss der Germanischen Philologie.

[4] In 1938, de Vries was admitted to the Royal Netherlands Academy of Arts and Sciences, and was appointed Chairman of its Folklore Committee.

In 1940, shortly after the Battle of the Netherlands, de Vries authored the pamphlet Naar een betere toekomst (Toward a Better Future), where he expressed his opposition to democracy and argued in favor of a German victory in the war.

In May–June 1948, the Bijzonder Gerechtshof found de Vries guilty of "intellectual collaboration", and deprived him of the right to vote and hold political office.

Thanks to his vast knowledge, familiarity with the source material of ancient Northern Europe, and abilities as a synthesizer, de Vries must be considered one of the greatest social scientists of his generation.

[8] After his release from prison, de Vries had no income, and was thus permitted to resume working as a secondary school teacher in Dutch literature in Oostburg.

[4][9][10] After his relocation to Utrecht in 1957, a number of important works were published, including Kelten und Germanen (1960), Keltische Religion (1961) and Forschungsgeschichte der Mythologie (1961).

[3] De Vries' publications on Old Norse literature, Dutch etymology and Germanic religion have formed the basis for modern research on the subjects, and have remained standard reference works up to the present day.

[5][4] Stefan Arvidsson describes de Vries as the greatest Germanic studies scholar of the 20th century, and as one of the foremost social scientists of his generation.