His father was married to one of the daughters of Jean Baptiste August Kessler, who helped create the company now known as Royal Dutch Shell; one of his uncles was Geldolph Adriaan Kessler, who helped create the Dutch steel industry.

During World War II, Kohnstamm and Kessler were both held hostage by the Germans along with other prominent Dutchmen at Beekvliet in Kamp Sint-Michielsgestel; they became quite close there despite the difference in age.

[2] Kohnstamm was educated at Amsterdam University, where he studied Modern History, before taking up a fellowship at American University, Washington, D.C. During 1938 and 1939 he travelled through the United States as part of his studies.

[3] His correspondence with his father during this period discussed his impressions of the United States and his concerns with the looming war.

He was Vice President of the Netherlands' Schuman Plan delegation in 1950, serving as Secretary to the High Authority of the European Coal and Steel Community from 1952 to 1956.