De Weldon first received notice as a sculptor at the age of 17, with his statue of Austrian educator and diplomat Professor Ludo Hartman.

De Weldon enlisted in the United States Navy during World War II and was discharged with the rank of Painter Second Class (PTR 2).

In 1951, De Weldon acquired the historic Beacon Rock estate in Newport, Rhode Island, where he lived until 1996 when he lost the property and most of his assets to financial hardship.

At the conclusion of the war in 1945, the Congress of the United States commissioned de Weldon to construct the statue for the Marine Corps War Memorial (called Iwo Jima Memorial) in the realist tradition, based upon the famous photograph of Joe Rosenthal, of the Associated Press agency, taken on February 23, 1945.

De Weldon made sculptures from life of three (on June 23, 2016, John Bradley was not believed to be in the photo)[4] of the six servicemen raising the replacement United States flag on Mount Suribachi, on Iwo Jima.

De Weldon also contributed in creating Malaysia's Tugu Negara (National Monument) when the country's first Prime Minister, Tunku Abdul Rahman saw the Marine Corps War Memorial statue in his visit to America in October 1960 and personally met him for favour to design the monument.