The Platonic Philosophy pleased him extremely, and he retained a liking to it all his life: he had read all the books of the sect, had commented their works, and knew them almost by heart.
The Law wholly took him up afterwards: he went to study it at the faculty of law at Orleans, the most celebrated for that science, and took the degree of Licentiate.

Returning home he followed the Bar; some time after, he was nominated Counsellor and Echevin: William prince of Orange made him Master of Requests.
The University of Leyden being founded in 1575, Cornelius de Groot resigned his post in the magistracy, to follow his ruling inclination of being useful to youth; and did not think it beneath him to accept of a Professor's place in the new University: he first taught Philosophy, and was afterwards made Law-professor; an employment that pleased him so much, he preferred it to a seat in the Grand Council at the Hague, which was several times offered him, but which he constantly refused.

His reputation was so great, the Grand Council often consulted with him on affairs of importance.

Six times he was honoured with the dignity of Rector, a place of great honour and authority: the members of the University, and all who are enrolled in the Rector's book, depend on his jurisdiction; before him their causes, civil and criminal, are brought, and from his sentence there is no appeal: a revisal of it is all that can be demanded.

Cornelius de Groot died without issue in the year 1610, on the same day of the month of July on which he was born.