One of six children, Willem grew up in Amsterdam where his parents were theater costume designers. When Willem was 17, he fought with his parents about his homosexuality. He left home and severed contact with his family. He began writing and painting, and in the 1920s was commissioned to do a mural for the Rotterdam town hall. In 1932 he moved to the countryside near Apeldoorn.

1933-39: When he was 38, Willem met Jan Tijssen, the son of a greengrocer, and they lived together for the next seven years. Although he was a struggling painter, Willem refused to go on welfare. In 1938 Willem began writing a biography of Dutch painter Matthijs Maris, and after the book was published, Willem's financial situation improved.

1940-44: The Germans invaded the Netherlands in May 1940. Soon after the occupation, Willem joined the resistance. His unit's main task was to falsify identity papers for Dutch Jews. On March 27, 1943, Willem's unit attacked the Amsterdam registry building and set it on fire in an attempt to destroy records against which false identity papers could be checked. Thousands of files were destroyed. Five days later the unit was betrayed and arrested. That July, Willem and 11 others were executed.

Before his execution, Willem asked a friend to testify after the war that "homosexuals are not cowards." Only in the 1980s did the Dutch government posthumously award Willem a medal.