During the 1930s, after a lengthy period of training, Rowlett and his colleagues compiled codes and ciphers for use by the U.S. Army and began solving a number of foreign, notably Japanese, systems.

"[2] Rowlett supervised cryptanalyst Virginia Dare Aderholdt, who decrypted the Japanese surrender message, August 14, 1945.

[3] Rowlett also played a crucial role in protecting American communications during World War II, making fundamental and innovative contributions to the design of the SIGABA cipher machine.

In addition to having highly developed cryptanalytic skills, Rowlett was a good manager, and he rose quickly within the organization.

In 1965 he was awarded the National Security Medal by President Lyndon B. Johnson for his work on breaking the Japanese Purple cipher.