Van Eysinga worked in the Dutch Ministry of Foreign Affairs from 1902 to 1908, eventually rising to Director of the Legal and Political Section.

He served as an assistant delegate to the 1907 Hague Convention, and in 1910 was appointed the Dutch representative on the Central Commission for Navigation on the Rhine, a position he held until 1930.

[3] After World War I, Van Eysinga served as a member of the Dutch delegation to the 1919 Paris Peace Conference, where he took part in the negotiations leading to the Covenant of the League of Nations and portions of the Treaty of Versailles relating to the Rhine.

In 1921, van Eysinga served on an arbitral panel led by Max Huber, tasked with settling a dispute between Germany and certain Dutch banks.

Van Eysinga was the Dutch government's representative at the 1930 League of Nations Codification Conference, which purported to codify certain aspects of international law.