Mervyn was born on the 5th of May, 1887 in Vernon, Utah Territory. His grandfather had immigrated to Utah from Wales with Mormon pioneers and he had established cattle operations near Taylorsville. When Mervyn received his acceptance to the US Naval Academy, he was living in Idaho and went on to graduate from the Academy third in his class in 1910. Mevyn's younger brother Howard graduated first in his class from the US Military Academy in 1912.

After graduating, Mervyn was assigned to the USS California and became an ordnance and gunnery specialist with the Ordnance Bureau at the Washington Naval Yard during World War 1. He received his first command on the USS Bernadou and later assumed command of the USS West Virginia on the 2nd of July, 1941. By this time, Mervyn was a captain and it was his actions during the Japanese attack on Pearl Harbor that would earn him the Medal of Honor. The citation reads:

For conspicuous devotion to duty, extraordinary courage, and complete disregard of his own life, above and beyond the call of duty, during the attack on the Fleet in Pearl Harbor, by Japanese forces on 7 December 1941. As Commanding Officer of the USS West Virginia, after being mortally wounded, Capt. Bennion evidenced apparent concern only in fighting and saving his ship, and strongly protested against being carried from the bridge.

Mervyn's posthumous Medal of Honor was presented to his wife, Louise, and she christened a US destroyer on the 4th of July 1943 in his honor. Mervyn Sharp Bennion is buried in the Salt Lake City Cemetery; West 2 148 1 Cent.