At the Barlaeus Gymnasium, he was not accepted as a member of the school association Disciplina Scipio Vitae because he was a Jew.

After a year of no notable action in 1920, in 1921, Wijnkoop interpellated against Van Karnebeek again, this time to provide support to the starving workers and peasants in Soviet Russia.

Then, in 1922 and 1924, he showed further Communist sympathies by pressuring minister of the Dutch government Simon de Graaff of the persecution against Communists in Dutch East Indies, and against minister Gerardus Jacobus van Swaaij on the pay cut for railway staff, respectively.

In 1924, he pressured de Graaff again, this time after the government of the British Raj announced measures to combat the popular movement of workers and peasants in India.

After going without much notable action for about 4 years, he came up again, this time against Prime Minister Charles Ruijs de Beerenbrouck about the violent clashes between police and workers following a strike in a zinc oxide factory in Maastricht.

In 1907 Wijnkoop founded, together with Jan Cornelis Ceton and Willem van Ravesteyn, a magazine, called The Tribune.

On Prinsjesdag 1932, he and fellow Communist Louis de Visser interrupted the Throne Speech from Queen Wilhelmina by shouting.

While in hiding from the Germans during World War II, Wijnkoop suffered a fatal heart attack.

Wijnkoop's widow, Johanna van Rees, was angry Louis de Visser showed up at the funeral while he was wanted, but still asked him to make the graveside speech.