He was born in Rotterdam in the Netherlands. His father was also named Hendrik Willem van Loon. His mother was named Elisabeth Johanna Hankin. In 1902, he went to the United States to study at Cornell University. He graduated from Cornell in 1905. In 1906 he married Eliza Ingersoll Bowditch. Her father was a teacher at Harvard. They had two sons, Henry Bowditch van Loon and Gerard Willem van Loon. After they got married, Hendrik and Eliza moved to Munich, Germany. Hendrik studied at the University of Munich. In 1911 he got his Ph.D. He had to write a long paper to show that he was a good enough student to get the Ph.D. The paper he wrote was called "The Fall of the Dutch Republic". In 1913 he published it as a book. During the Russian Revolution of 1905 and again at the start of World War I he worked for the Associated Press. He went to the places where soldiers were fighting each other and wrote about what the battles were like. From 1915 to 1917 he taught at Cornell University. In 1919 he was made a citizen of the United States. In 1920 he married Eliza Helen (Jimmie) Criswell. In 1927 he married Frances Goodrich Ames, a woman who wrote plays. Later he stopped being married to Frances Ames. He went back to Eliza Criswell. No one knows if they got married again. When he died in 1944, he gave his things to Eliza Criswell.