Buckles -- who also survived 3 1/2 years in a Japanese prisoner of war camp during World War II -- died of natural causes at his home in Charles Town, W.Va., biographer and family spokesman David DeJonge told The Journal of Martinsburg, W.Va.
The veteran celebrated his 110th birthday earlier this month, but his family said that his health had been deteriorating since late last year. He died Sunday.
"I knew what was happening in Europe, even though I was quite young," he told The Washington Post in 2005. "And I thought, well, 'I want to get over there and see what it's about.'"
But the 16-year-old struggled to find a service that would accept him. "I went to the state fair up in Wichita, Kan., and while there, went to the recruiting station for the Marine Corps," Buckles told The Associated Press in 2007. "The nice Marine sergeant said I was too young when I gave my age as 18, said I had to be 21."
He returned to the recruiting station a week later, this time claiming he was 21. "I passed the inspection," he told AP, but the recruiting officer "told me I just wasn't heavy enough." Next Buckles tried the Navy but was turned away because he was flat-footed.
Finally, he walked into the Army recruiting in Oklahoma City, where a captain demanded to see his birth certificate.
"I told him birth certificates were not made in Missouri when I was born, that the record was in a family Bible," Buckles said. "I said, 'You don't want me to bring the family Bible down, do you?'" Buckles said with a laugh. "He said, 'OK, we'll take you.'"
In December 1917, his army detachment set sail for Europe on the British liner Carpathia, which five years earlier had rescued survivors of the Titanic. When he arrived in England, the young Buckles pestered senior officers to repost him to the front. "A knowledgeable old sergeant said if you want to get to France right away, go into the ambulance corps," he said in a 2001 interview with the Library of Congress.
Buckles was moved to France but never served on the front line. At the end of the conflict -- some 8.5 million people had died by the time the shooting stopped on Nov. 11, 1918 -- his unit helped exhausted and thankful German prisoners of war return home. One handed him a souvenir, a belt with a buckle inscribed, "Gott Mit Uns" (God with us), which still sits in his farm house, reported USA Today.
Like many of the more than 4 million Americans who served in WWI -- about 116,500 died in the conflict -- Buckles received little help when he arrived back the U.S. He was handed a $60 bonus by the government, and the local YMCA offered him one month of free membership. Buckles put the bonus toward typing and shorthand classes, and he found a clerical job with a steamship company.
He sailed the world during the Depression years and finally, 24 years after leaving the army, found himself on the front line. He was representing a U.S. shipping company in Manila in January 1942 when the Philippines' capital was captured by invading Japanese troops. Buckles was held at a succession of prison camps in the Philippines, including the notorious facility in Los Banos, where foreign internees were regularly beaten by Japanese guards and fed meager, grub-filled rations.
"The starvation at Los Banos was so bad, it is surprising that any of us survived," he wrote in 2009. "When The 11th Airborne finally freed us on February 23, 1945, we all looked pretty much like skeletons with skin on."
Buckles turned 44 that year. Suffering from beriberi and dengue fever, he decided that he'd had enough excitement abroad. He worked as a salesman for a paint company and married Audrey Mayo in 1946. Eight year later, the couple bought a 330-acre cattle farm near Charles Town.
Audrey died in 1999, the same year that French President Jacques Chirac awarded Buckles the French Legion of Honor, the country's highest decoration. His wartime exploits received a renewed flurry of interest at home following the death of 108-year-old Harry Landis on Feb. 4, 2008 -- leaving Buckles as the war's last living U.S. veteran.
"I always knew I'd be one of the last because I was one of the youngest when I joined," Buckles, then 107, told the New York Daily News. "But I never thought I'd be the last one."
Buckles is survived by his daughter, Susannah Buckles Flanagan. With his death, only two of the 65 million people mobilized during the Great War are known to be alive, The Washington Post reports: an Australian man, 109, and a British woman, 110.