George M. Elsey, who served in the White House under presidents Franklin D. Roosevelt and Harry S. Truman. (Courtesy of the family)

George M. Elsey, one of the last living links to the presidency of Franklin D. Roosevelt, who pushed the president’s wheelchair around the White House’s top-secret Map Room during World War II and who later served as a top adviser, speechwriter and political strategist to President Harry S. Truman, died Dec. 30 at an assisted-living center in Tustin, Calif. He was 97.

The cause was prostate cancer, said his daughter, Anne Elsey Kranz.

Mr. Elsey, who also served as president of the American Red Cross from 1970 to 1983, was a young naval officer assigned to the top-secret White House intelligence office during World War II. He observed military strategy sessions and attended international conferences with Roosevelt and Truman, and may have been the last person to have been acquainted with the two presidents, British statesman Winston S. Churchill and Soviet leader Joseph Stalin.

For good measure, Mr. Elsey was at Omaha Beach as a naval officer and historian on June 6, 1944, better known as D-Day. He was with Truman in 1945, when the president approved the use of atomic weapons in the waning days of World War II. He later decoded the official message announcing that an atomic bomb had been dropped over Hiroshima, Japan, and hand-delivered it Truman.

Mr. Elsey was 24 when he went to work in the White House Map Room in 1942. Security was so tight that Roosevelt’s Secret Service agents were not allowed inside. Not even the vice president could enter the room, which held records of presidential communications with other heads of state and the country’s most sensitive military plans.

“The walls were covered with maps and charts,” Mr. Elsey said in a 2005 interview with NPR. Pins marked the locations of ships at sea; troop movements were recorded in erasable grease pencil.

“All the messages between Roosevelt and Churchill and Stalin were delivered to us,” Mr. Elsey added. “We also were privy to the plans for future operations. Our safes held plans for the Normandy invasion as it was being developed. So it was a very secret place, a fascinating place to be assigned.”

To accommodate Roosevelt and his wheelchair, desks and chairs were placed in the center of the room. Mr. Elsey often pushed the president around the perimeter, as he peered at the maps detailing the progress of the war.

“I didn’t know until I actually met him that he was totally unable to move on his own,” Mr. Elsey recalled.

Churchill, Britain’s prime minister throughout most of World War II, visited the White House several times during the war. He sometimes came into the Map Room late at night, smoking his cigar, when Mr. Elsey was one of the few officers on duty.

In May 1943, Roosevelt, Churchill and several top military officials descended on the Map Room after receiving an urgent communique from Stalin. The Soviet leader was asking the United States and Britain to mount a western offensive against the German regime to ease Nazi pressure on the Russian front.

“The give-and-take that night across the Map Room’s navy-gray steel desks was vigorous,” Mr. Elsey wrote in a 2005 memoir, “An Unplanned Life.”

He stood by as the Allies’ top leaders debated a strategy that would result in the invasion of Normandy a year later.

“Here I was, a kid, listening to Roosevelt, Churchill and the Joint Chiefs of Staff arguing over the principal strategy of World War II,” Mr. Elsey told the Orange County Register in 2013.

No longer in the White House in June 1944, Mr. Elsey found himself in Britain as a naval intelligence officer and historian. He went across the English Channel and arrived on Omaha Beach soon after the D-Day landings.

He later returned to his White House post and, following Roosevelt’s death on April 12, 1945, helped introduce Truman to the Map Room. Truman was not aware of the secret Manhattan Project to build atomic weapons until he was sworn in as president.

In July and August 1945, Mr. Elsey accompanied Truman to the Potsdam conference in Germany, where the new president, Churchill and Stalin negotiated settlement terms of the war. Mr. Elsey watched Truman write a message to Secretary of War Henry L. Stimson granting permission to use atomic weapons against Japan: “Suggestions approved. Release when ready but not sooner than August 2” — the final day of the Potsdam conference.

The bombs were dropped on Hiroshima on Aug. 6 and on Nagasaki three days later, causing horrific destruction but bringing an immediate end to the war.

“There was almost no decision for President Truman to make,” Mr. Elsey told Newsday in 1995. “From the beginning, it was assumed that, if this new weapon worked, it would be used. Well, it worked.”

George McKee Elsey was born Feb. 5, 1918, in Palo Alto, Calif., and grew up in Oakmont, Pa. His father was a chemist for Westinghouse and had a role in the Manhattan Project, but father and son could not discuss their classified work with each other until after the war.

Mr. Elsey graduated with highest honors from Princeton University in 1939 and received a master’s degree in history from Harvard University in 1940. He was studying for a doctorate in history when he entered the Navy Reserve.

After World War II, he was a top assistant to presidential adviser Clark Clifford. In 1948, Mr. Elsey helped draft a notable civil rights address that Truman delivered to Congress and later worked on executive orders that banned segregation in the military and in federal civil service jobs.

While traveling with Truman’s nationwide whistle-stop presidential campaign of 1948, Mr. Elsey contributed to speeches and informal talks, as Truman scored an unexpected victory over Republican Thomas E. Dewey.

“Truman was impossible not to like,” Mr. Elsey said in 2013. “Roosevelt was up in the clouds looking down on us. Truman was right down on the ground with us.”

After leaving the White House in 1951, Mr. Elsey worked under W. Averell Harriman at the short-lived Mutual Security Administration, coordinating foreign aid programs. He became an official of the American Red Cross in 1953.

In the 1960s, Mr. Elsey directed the Washington office of Pullman Inc., an international construction and engineering company, then was a special assistant to Clifford, who was secretary of defense in 1968 and 1969.

As president of the American Red Cross from 1970 to 1983, Mr. Elsey oversaw a fourfold increase in the charity’s income, and expanded operations for blood donations and medical services. He was an official with the International Federation of Red Cross and Red Crescent Societies from 1983 to 1987 and served on the boards of several companies and nonprofits, including the National Geographic Society, Brookings Institution and Harry S. Truman Library Institute. He was president of the White House Historical Association from 1990 to 1996.

Mr. Elsey lived in Washington for more than 60 years before moving to Irvine, Calif., in 2005.

His wife of 52 years, the former Sally Phelps Bradley, died in 2004. Survivors include two children, Anne Elsey Kranz of Tustin, Calif., and Howard McKee Elsey II of Laguna Hills, Calif.; and two grandsons.

When Mr. Elsey left the White House in 1951, Truman urged him to write a book about his experiences.

“When the time comes,” Truman said, according to contemporary accounts, “I think that you will be in a position to make a unique contribution to the history of these times.”

Mr. Elsey’s memoir was published in 2005. A paperback edition of “An Unplanned Life” is scheduled to be released later this year by the University of Missouri Press.