He became a full-time Assistant in the Department of Mathematics of the Technological University of Delft in September 1939 while continuing his studies.

He received his PhD in 1943 from the Vrije Universiteit Amsterdam with a thesis entitled "Over modulaire vormen van meer veranderlijken" advised by Jurjen Ferdinand Koksma.

[2] De Bruijn started his academic career at the University of Amsterdam, where he was Professor of Mathematics from 1952 to 1960.

He is especially noted for: He wrote one of the standard books in advanced asymptotic analysis (De Bruijn, 1958).

In the late sixties, he designed the Automath language for representing mathematical proofs, so that they could be verified automatically (see automated theorem checking).