His ROTC experience helped him absorb the details and nuances of modern warfare and spycraft, as he voraciously read books on the military and unclassified government documents, which would surface 15 years later when he wrote his first novel, "The Hunt for Red October," published in 1984.
He graduated in 1969 with an English degree and moved to Hartford, Conn., where he worked for an insurance company from 1969 to 1973. That year, he returned to Maryland and joined O.F. Bowen, his in-laws' insurance agency in the Calvert County community of Owings.
His insurance office had a number of military clients, which kept him around epaulets and brass buttons. He wrote an article in 1982 on the MX missile system for Proceedings, a publication of the Naval Institute in Annapolis.
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His first publisher, Jim Barber, a retired Navy captain and retired executive director of the Naval Institute, once recalled Mr. Clancy as "a very bright guy who knows clearly what he thinks and doesn't hesitate to let you know what he thinks. ... You don't have any problem understanding where you stand with him."
Bored with the insurance business, Mr. Clancy began working on a novel in his spare time, after reading a newspaper account of a mutiny aboard the Soviet destroyer Storozhevoy in 1975. The result was "The Hunt for Red October," a tale of superpower conflict centered on a renegade Soviet nuclear submarine.
The book took off like a heat-seeking missile, selling 300,000 hardbacks and 2 million paperbacks in two years.
The book got a significant boost when President Ronald Reagan declared it a "perfect yarn" and other officials hinted playfully that it might contain classified information. One version of that story has a publicist working feverishly to get the book to the presidential bedside table, but Mr. Clancy insisted it was simply a reviewer with a friend with connections who passed it along.
"I've been lucky," Mr. Clancy said in the 1992 interview with The Sun, as he inhaled an omnipresent Merit menthol cigarette.
In 1985, Mr. Clancy told The Sun that he wasn't trying to write the great American novel.
"I did not write King Lear. I am not Hemingway, Faulkner or Shakespeare, and I won't say Steinbeck, because I don't like him," he said.
"The whole point of writing is to get an idea out of your head and put it in somebody else's head," he told the newspaper in 2004.
Mr. Clancy's daily writing regimen was to work four to six hours with a goal of producing at least five pages.
Mr. Clancy's success enabled him to purchase a 17,000-square-foot penthouse condominium at the Ritz-Carlton whose price tag was $16.6 million and where he purportedly wanted to build a firing range.
The Sun reported last year that Mr. Clancy's Inner Harbor digs had by far the highest tax payment of any home in Baltimore — almost $350,000. That was more than all the payments from homeowners in some city neighborhoods, according to the article.
He also owned an 80-acre farm overlooking the Chesapeake Bay in Southern Maryland that included a retired Army Sherman tank, a gift from his first wife.
When his third novel, "Patriot Games," was published in 1987, again featuring the character of Jack Ryan, a Sun reviewer wrote, "Mr. Clancy catches the smells of the Chesapeake Bay country like John Barth and the color and activity of Baltimore and its environs like Anne Tyler. ... Where alas, is a Washingtonian to enshrine that city with the loving felicity of this trio?"
Some observers thought that with the fall of communism in 1991, Mr. Clancy, who often used coups in his books as a literary device, might have trouble continuing to write thrillers.
Later that year, with the publication of "The Sum of All Fears," he turned to Middle East nuclear terrorists and proved his versatility, giving agent Jack Ryan a further lease on life.
As part of a divorce settlement in 1999 from his first wife, the former Wanda Thomas, Mr. Clancy, the second-largest Orioles shareholder, split his ownership, about 24 percent before the settlement, with his ex-wife, The Sun reported in 2000.
Mr. Clancy later served as the Orioles' vice chairman of community projects and community affairs.