Riordan had written a series of mystery novels for adults before he began work on The Lightning Thief, the first Percy Jackson book. The story began as a series of bedtime stories for his son Haley, then in second grade, about the adventures of ancient Greek gods. Because Haley had been diagnosed with ADHD and dyslexia, Riordan decided that his hero should face the same challenges. He told Haley the story of Percy Jackson and his travels across the United States in search of Zeus's lightning bolt. Haley suggested that he should turn that story into a book, and Riordan wrote The Lightning Thief over the following year.

Since then, the Percy Jackson books have inspired further series, short stories, and graphic novels as well. And, in series like Magnus Chase and the Kane Chronicles, Riordan has continued to explore his favorite theme: teens tangling with immortals, with the fate of the world (or sometime the universe) hanging precariously in the balance.