CPL Yarbrough joined the Marines as a volunteer on June 1941. He had been stationed in the Hawaiian Islands before the Pearl Harbor attack. His last message home was a brief letter and money order, mailed on August 4, 1942, and reaching his hometown on August 13, four days before he died, a day after turning 21, in the Makin Island Raid. The money order was to purchase a new pair of spectacles for his mother.

Mason Opel ‘Slim’ Yarbrough was born on August 17, 1921, In Casey, Arkansas, to Florence, Missouri Hubbard, and James Tecumseh Yarbrough. Mason was preceded by Maria, his brother Granville, called ‘Ted’ and followed by sisters Lottie and Rosella. James, the father, provided for his family as a brick Mason, and Florence kept the house and raised her brood. Mason and his siblings attended school in Arkansas.

At some point, the family moved to Sikeston, Missouri, where their father worked construction, and the Yarbrough children continued their schooling, attending the Hunter and Lennox schools near Sikeston, Missouri. The 1940 US Census indicates that Mason Yarbrough only went as far as seventh grade before he left school. He had been in the Civilian Conservation Corps in Idaho when he entered the Marines with Edward Palmer, his best friend.

War had been raging on the European continent since September of 1939 and longer in Asia. When war came to the United States, it first came from the East, with the surprise bombing of the Pacific Fleet at Pearl Harbor by the Japanese Imperial Navy on December 7, 1941, a Day of Infamy. Mason Yarbrough already a Marine since June, graduated from boot camp at San Diego and was a Private First-Class. A year later he was promoted to Corporal.

In 1942, a new Marine Corps commando force, the Marine Raiders, was organized. Under Evans Carlson, CPL Yarbrough and his friend joined the new commando force, the 2nd Raider Battalion. The Raiders were divided into Edison's Raiders of the 1st Marine Raider Battalion and Carson's Raiders of the 2nd Marine Raider Battalion. Mason Yarbrough and his friend were assigned to the Company B, 2nd Marine Raider Battalion. On August 8, 1942, Raider Companies A and B boarded the USS Argonaut (SS-1) and the USS Nautilus SS-168; they later arrived at Butaritari, Makin Atoll, Gilbert Islands, on August 16, 1942.

On August 17, the Raiders came ashore on Makin from the submarines in rubber rafts fitted with outboard motors. The Marines goal was to divert attention from the Allied landings on Guadalcanal and Tulagi. The landings were unopposed, but while the two companies were forming upon the beach, one trooper accidently discharged his rifle, giving the alarm to the enemy.

The Raiders destroyed installations and disrupted Japanese reinforcements bound for the island of Guadalcanal. Yarbrough and the Raiders neutralized 83 of the enemy and two flying boats. Soon, the Marines saw the Japanese coming, and the two sides fired at each other over a distance of 30 yards. "It was just two skirmish lines across the island, from palm tree to palm tree, in a big military version of the OK Corral," said SGT Buck Stidham, a fellow Raider.

It was during this assault that CPL Mason Yarbrough was killed in the line of duty.

CPL Yarbrough had just given his friend Eddie Palmer a cigarette when he was killed by a burst of machine gun fire. Palmer was also wounded and said later that Yarbrough was killed instantaneously.

The Marines were forced to evacuate Makin on August 18 and leave their dead behind in the face of heavy enemy reinforcements. Marine casualties from the two days' fighting totaled 51, with 19 killed, 14 wounded, 12 missing, and seven presumed drowned. The missing totaled twelve, of which nine were known to have survived and were taken to Kwajalein, where they were beheaded.* The Gilbert Islanders buried the 19 Marine dead, which included Yarbrough, in an unmarked group grave. Mason’s name would be added to the Wall of the Missing at the Punchbowl National Cemetery in Hawaii.

At the end of 1999, The Central Identification Laboratory (CILHI), based at Hickam Field, Hawaii, recovered the remains of 19 Marine Raiders killed in August 1942 during the raid on Makin Atoll. This recovery ended a search that began in 1948 by the Graves Registration Team, looking for CPL Yarbrough and the other 18 Marines left behind on Makin Atoll. Through November and December of 1999, a mass grave was discovered using radar and cadaver dogs. Human remains, equipment, and ID tags were found. A Gilbert Islander, Bureimoa Tokarei, who was 16 and helped bury the Marines, led the CILHI team within feet of the burial site.

The repatriated remains were returned to the States. The families of the 19 Makin Marines decided on a joint internment at Arlington. On August 17, 2001, CPL Mason O Yarbrough and 18 fellow Marine Raiders were buried in a group grave with full military honors in section 60 grave, 8036, at Arlington National Cemetery on the 59th Anniversary of the Makin Raid.

After Yarbrough 's remains were identified, a rosette was added to his name at the Punchbowl, indicating he was accounted for. His remains were also buried December 15, 2000, with his parents in Sikeston Memorial Park in Sikeston, Missouri.