He was born in Dublin and educated at Synge Street CBS, Portobello. He was an altar-boy at St. Kevin's Church (Roman Catholic), Harrington Street and studied the violin at the Irish Academy of Music in Dublin. Ryan moved to London in 1940, and became a war correspondent for The Daily Telegraph in 1941.

He initially covered the air war in Europe, flew along on fourteen bombing missions with the Eighth and Ninth United States Army Air Forces (USAAF), and then joined General George S. Patton's Third Army and covered its actions until the end of the European war. He transferred to the Pacific theater in 1945, and then to Jerusalem in 1946.

Ryan emigrated to the United States in 1947 to work for TIME, where he reported on the postwar atomic testing done by the United States in the Pacific. Then reported for TIME on the Israeli war in 1948. This was followed by work for other magazines, including Collier's Weekly and Reader's Digest.

He married Kathryn Morgan (1925—1993), a novelist, and became a naturalized citizen of the United States in 1951.

On a trip to Normandy in 1949 he became interested in telling a more complete story of Operation Overlord. He began compiling information and conducting over 1000 interviews as he gathered stories from both the Allies and the Germans, as well as the French civilians.

Ryan's 1957 book One Minute to Ditch! is about the successful ocean ditching of a Pan American Boeing 377 Stratocruiser. He had written an article about the ditching for Collier's in their December 21, 1956, issue and then expanded it into the book.

In 1956 he began to write down his World War II notes for June 6, 1944 D-Day, which tells the story of the D-Day Invasion of Normandy, published three years later in 1959. It was an instant success, and helped work on the screenplay for the film of the same name, for which Darryl F. Zanuck paid the author $175,000 for the screen rights to produce the film.

His next work was The Last Battle (1966), about the Battle of Berlin. The book contains detailed accounts from all perspectives: civilian, American, British, Russian and German. It deals with the intense political situation in the spring of 1945 in which the eastern and western fronts fought for the chance to liberate Berlin and carve up the remains of Germany.

Then A Bridge Too Far (1974), which tells the story of Operation Market Garden, the ill-fated assault by airborne forces on the Netherlands culminating in the Battle of Arnhem. This work was also made into to major motion picture of the same name.

He was awarded the French Legion of Honor, and an honorary Doctor of Literature degree from Ohio University, where the Cornelius Ryan Collection is housed (Alden Library).

Ryan found out he had cancer in 1970, and struggled to finish the book along with his illness until his death in Manhattan, while on tour promoting the book, A Bridge Too Far, only two months after publication.

Four years after his death, his struggle with cancer was detailed in A Private Battle, written by his wife, from notes he had secretly left behind for that purpose. He is buried in the Ridgebury Cemetery, in northern Ridgefield, Connecticut.