He was 110.
The indomitable Frank Buckles, who also survived three years in a Japanese prisoner of war camp, “died peacefully” of natural causes at his home in Charles Town, West Virginia, a family spokesman said.
Buckles celebrated his 110th birthday on February 1, but his family said his health had been failing since late last year.
He spent the last years of his life campaigning for a national WWI memorial in Washington, testifying before a Senate panel in December 2009 on the matter.
Buckles “actively pursued his role as the torchbearer for his fellow soldiers from the Great War”, the spokesman David DeJonge said.
Born Frank Woodruff Buckles in Missouri in 1901, Buckles rushed to enlist when the US entered the war in April 1917 after reading about the conflict in the newspapers.
The marines and the navy turned him down because he was only 16, but Buckles managed to convince the army recruiter he was 21.
Some 4.7 million Americans fought in World War I, the vast majority in the American Expeditionary Force that sailed to Europe.
When the Americans entered the war, begun in 1914, Russia was on the verge of collapse, thousands had been slaughtered in the trenches on the western front, mutinies were breaking out in the French army and German submarines were taking a heavy toll on allied shipping. By the time the shooting ended on November 11, 1918, some 8.5m people had died.
But Buckles, who rose to the rank of corporal, never made it to the front lines in France and after the war, his unit escorted prisoners back to Germany.
By the time he left the army in 1920, there was wide disillusionment back home and a feeling that perhaps the United States had been tricked into the war.
Belatedly on the front line, Buckles was captured by invading Japanese soldiers as a civilian after the shipping freighter he was working on was captured in Manila in January 1942, just weeks after the attack on Pearl Harbour brought the United States into World War II.
He spent the next three years in a prison camp.
After the war, Buckles returned to the US, married and bought a cattle farm in West Virginia.
“Nobody here knew that I had been in World War I until quite recently,” he told the Library of Congress.
That was until French president Jacques Chirac awarded him the Legion of Honor (Legion d’honneur) medal at in 1999.
In March 2008, Buckles was honoured at a special ceremony at the Pentagon and the White House by president George W Bush.
His wife Audrey died in 1999 and he is survived by his daughter Susannah.