He received his country's highest award for bravery during combat, the U.S. Medal of Honor, for his "extraordinary heroism" for taking the initiative, during a critical stage of the Battle of Five Forks on April 1, 1865, to lead a successful advance upon the enemy while fighting with the 1st West Virginia Cavalry.

[7][8][9] During the summer of the second year of the American Civil War, Wilmon Blackmar left Phillips Exeter to enlist for military service.

[16][17] With the Appomattox Campaign successfully concluded and the war officially declared over, Blackmar honorably mustered out at the rank of captain on July 8, 1865.

Admitted to the Suffolk Bar following his graduation, he served as a lawyer during the 1880s and 1890s, a fact confirmed by his 1894 application for a United States passport, which also noted that he was a permanent resident of Hingham, Massachusetts who was a 53-year-old man who was 5'7" tall with dark hair and blue eyes with an oval face with a high forehead, straight nose, round chin, and fair complexion.

[20] Upon retirement, he served as Judge Advocate General for several Massachusetts governors, and was also elected Commander-in-Chief of the Grand Army of the Republic in 1904,[21] a position he continued to hold until his death.

[23] He died in Boise, Idaho, on July 16[16] while engaged in a lengthy series of visits to Grand Army of the Republic posts which he had scheduled in his capacity as the G.A.R.

[22][25] When it was read in August 1905, his will revealed that he had left $3,000 to Lexington, Kentucky resident Nancy T. Creel, the daughter of the nurse who had rendered care to Blackmar during the American Civil War as he recuperated from typhoid fever.

Citation:[3] The President of the United States of America, in the name of Congress, takes pleasure in presenting the Medal of Honor to Lieutenant Wilmon Whilldin Blackmar, United States Army, for extraordinary heroism on April 1, 1865, while serving with Company H, 1st West Virginia Cavalry, in action at Five Forks, Virginia.