He is the head of the type foundry Fontfabrik, also trading as LucasFonts. He is mostly known for the very large font family Thesis (TheSans, TheSerif, TheMix, TheSansMono and later TheAntiqua), Calibri, the longtime default font with Microsoft Office, and Consolas. De Groot has also designed various custom fonts for particular clients: Corpid (previously AgroSans) for the Dutch Ministry of Agriculture.
SunSans for Sun Microsystems.
SpiegelSans and Taz for, respectively, the German magazines Der Spiegel and die tageszeitung. And FolhaSerif for the Brazilian newspaper Folha.
Some of these have since become publicly available. De Groot teaches at the Design Faculty of the University of Applied Sciences, Potsdam, Germany.
TAZ for the newspaper die Tageszeitung