Martin James O'Brien was born on January 2, 1918, in Worcester, Worcester County, Massachusetts. He was the son of Gertrude E. Deady O'Brien. He was married to Dorothea McNamara.

Martin graduated in 1941 and went to work for the Norton Company of Worcester. In February 1943, he left to enter the Marines. He served in Company E, 2nd Battalion, 3rd Marine Regiment, 3rd Marine Division as a Second Lieutenant during World War II. Ordered to attack Japanese positions at the crest of Chonito Ridge on July 22, O'Brien led his platoon in a frontal assault that was halted halfway by ferocious hostile machine-gun fire. O'Brien withdrew his men to safety, reassembled, and led a second attack. Despite machine-gun, rifle, grenade, and mortar fire, Martin O'Brien reached the crest of the ridge before he was mortally wounded. He was Killed in Action on July 23, 1944, during the Battle for Guam.

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