Career
During the Second World War, he was a prisoner in several Japanese prisoner-of-war camps, like Sukamiskin Penitentiary in Bandung. Between 1950 and 1969 he was mayor of Wageningen. As a senator, he often collided with Minister of Foreign Affairs Joseph Luns on the issue of Netherlands New Guinea.
In 1969, he replaced Jannes Pieter Mazure as president of the Senate.
In 1973 De Niet was succeeded by Theo Thurlings.