Following the United States entry into the World War I in April 1917, Guymon decided to enlist in the Marine Corps as private during the same month and was ordered to the boot camp at Quantico, Virginia.

Guymon, who had in the meantime been promoted to sergeant, took part in the battle of Belleau Wood in June 1918, where he distinguished himself by delivering a message while under heavy machine gun fire.

[1] He was promoted to the temporary rank of second lieutenant in December 1918 and received French Fourragère and Croix de Guerre with Guilt Star for bravery by the Government of France.

Guymon sailed to Haiti in July 1924 with the 1st Marine Brigade under the command of Brigadier General Ben H. Fuller and participated in the skirmishes with Haitian rebels called "Cacos".

Guymon was promoted to the rank of first lieutenant and assumed temporary duty with Aircraft Squadrons, 2nd Marine Brigade under Brigadier General Logan Feland in December 1927.

He subsequently sailed for Nicaragua to support Marine units in the combats with Sandino bandits as pilot of Vought O2U Corsair with Franklin G. Cowie (later Brigadier general) as observer.

[1][3] His official Navy Cross citation reads: The President of the United States of America takes pleasure in presenting the Navy Cross to First Lieutenant Vernon M. Guymon (MCSN: 0-2472), United States Marine Corps, for distinguished service in the line of his profession as an airplane pilot attached to the Aircraft Squadrons, Second Brigade, U.S.M.C., operating against hostile bandits in the Republic of Nicaragua from 31 December 1927 to 26 May 1929.

During this period First Lieutenant Guymon performed 1021 hours of flying time over heavily wooded and mountainous country in fair weather and foul for the purpose of bringing supplies to and removing the sick and wounded from distant and isolated outposts.

The Herbert Schiff trophy is awarded to the naval aircraft squadron or unit having to its credit the most flying hours free from accident to personnel or damage to material.

He graduated in June 1935 along with some later famous general officers Muir S. Fairchild, Barney M. Giles, Haywood S. Hansell, Laurence S. Kuter, Lawson H. M. Sanderson, Hoyt Vandenberg or William J.

[citation needed] Guymon was then transferred to Marine Corps Air Base Kearny Mesa near San Diego in July 1943, before sailed to the Pacific area during the same month.

[1] Colonel Guymon served as chief of staff until November 1943, when he assumed command of Marine Aircraft Group 12 and led his unit during the air operations during the Emirau campaign and in the Solomon Islands.

Guymon remained in this capacity until the end of War and served twice as the representative of this command in connection with postwar operations and employment of the Marine Corps Air Station Ewa, Hawaii and later in the Marianas area.

He participated in the air operations during the Chinese civil war and was decorated with the Order of the Cloud and Banner with Special Rosette (6th Class) by the Government of Republic of China.