 De Groot was born at Garrelsweer, a tiny village in the municipality of Loppersum, Groningen, on May 7, 1914.He did both his undergraduate and graduate studies at the Rijksuniversiteit Groningen, where he received his Ph.D. in 1942 under the supervision of Gerrit Schaake.He studied mathematics, physics, and philosophy as an undergraduate, and began his graduate studies concentrating in algebra and algebraic geometry, but switched to point set topology, the subject of his thesis, despite the general disinterest in the subject in the Netherlands at the time after Brouwer, the Dutch giant in that field, had left it in favor of intuitionism.