Yardley tried at several other ventures, including selling invisible ink to government agencies but once again returned to writing, authoring a manuscript called Japanese Diplomatic Secrets, but the book was seized by federal authorities before it reached the printing press. He next wrote of espionage-oriented comedy called The Blonde Countess which was later made into a popular movie starring William Powell and Rosalind Russell. He also penned another novel called The Red Sun of Nippon.

Although still ostracized in the U.S. intelligence community, he was welcomed in other countries. He Traveled to China where he worked with Morris "Two-Gun" Cohen. helping to coordinate intelligence matters under Chiang Kai-shek in China's was against Japan. He also was recruited by Canadian authorities to help them establish a code-breaking operation for the Canadian government.

After the United States entered World War II, Yardley returned to the United States where he obtained a job with the federal government but was kept out of cryptology matters and instead assigned to the Office of Price Administration.

After the end of World War II, Yardley, an avid poker player, wrote a book called The Education of a Poker Player. He died a year later, the father of modern cryptology.