Clancy's publisher, Penguin Group (USA), announced his death but did not give the cause.
Born in Baltimore on April 12, 1947, to a mailman and his wife, Clancy was fascinated by military history as a child. He entered Loyola College as a physics major but switched to English as a sophomore. He later said he wasn't smart enough for the rigors of science, though he clearly mastered it in his fiction.
After school, he worked in an insurance office that had military clients. By the early 1980s he had written a piece about the MX missile system that was published by the Naval Institute. Boredom with his job led him to try his hand at fiction.
In an interview with The New York Times in 1987, he explained that unclassified information can lead to insights about things that are classified.
"One of the reasons we are so successful is that we have a free society with open access to information," he said. "If you change that, if you try to close off the channels of information, we'll end up just like the Russians, and their society does not work. The best way to turn America into another Russia is to emulate their methods of handling information."
Clancy lived in rural Calvert County, Md., and in 1993 he joined a group of investors led by Baltimore lawyer Peter Angelos who bought the Baltimore Orioles. Clancy also tried to bring an NFL team to Baltimore in 1993 but later dropped out.
Clancy was married twice, to Wanda Thomas and then to Alexandra Marie Llewellyn, and is survived by his wife and five children, according to his publisher. The publisher had no immediate details on funeral arrangements.