Isaac Keesing started work at the Amsterdamsche Bank, and also wrote articles on financial and economic matters for various newspapers, including the New York Times.

He also established a Dutch language series of puzzle books, Denksport, first published in 1930, which have continued up to the present, now as part of the Keesing Media Group.

[2][3] Following the German occupation of the Netherlands in 1940, Isaac Keesing's brother Jacob and his family tried to escape to England by boat, but the attempt failed and they committed suicide.

Keesing and his family lived during the latter part of the Second World War in Washington, D.C., where he worked at the Dutch embassy and filed another patent, for a card index system.

In his absence the company had been maintained by his accountant, Peter Diesfelt [nl], but its output had been heavily biased towards German propaganda, and its premises had been looted.

Isaac and Leo Keesing re-established the company, and set up a subsidiary, ASSiMiL, producing language courses, many supported by gramophone records.