As he was growing up, Upton's family moved frequently, as his father was not successful in his career. He developed a love for reading when he was five years old. He read every book his mother owned for a deeper understanding of the world. He did not start school until he was 10 years old. He was deficient in math and worked hard to catch up quickly because of his embarrassment. In 1888, the Sinclair family moved to Queens, New York City, New York, where his father sold shoes. Upton entered the City College of New York five days before his 14th birthday, on September 15, 1892. He wrote jokes, dime novels, and magazine articles in boys' weekly and pulp magazines to pay for his tuition. With that income, he was able to move his parents to an apartment when he was seventeen years old.