He was appointed a U.S. Navy Dental Surgeon with the rank of Lieutenant, Junior Grade, on May 8, 1917. He was assigned duty with the 6th Marine Regiment on March 26, 1918. During the Battle of Belleau Wood, Osborne's unit participated in the advance on Bouresches, France, about a mile southeast of Belleau Wood. Osborne sought to aid the wounded during the battle and was killed while attempting to carry an injured officer to safety on June 6, 1918. He was posthumously awarded the Medal of Honor and the Distinguished Service Cross for his actions on that day.
Weedon Osborne was 25 years old at the time of his death. He was buried in Aisne-Marne American Cemetery, Belleau, Aisne, France.