Walter was born on Christmas day 1930, in Melrose, Massachusetts and went to school in Malden, Reading, and Walpole. He enlisted in the US Army a month before his 17th birthday and when his age was discovered, Walter was discharged in January of 1948. He then enlisted in the US Marine Corps two months later in Baltimore, Maryland, and attended recruit training at Parris Island. Upon completion, he sailed to China from San Francisco for duty with the 3rd Marines.

Returning to the States, Walter was stationed at Camp Pendleton, California, in June of 1949 and was then transferred to the Marine Barracks at the Naval Air Station in Seattle, where he was married to Elizabeth Hickling. He reenlisted on the 20th of July, 1950, before returning to Camp Pendleton and deploying to Korea. It was his actions during the Battle of Inchon that would posthumously earn him the Medal of Honor. The citation reads:

For conspicuous gallantry and intrepidity at the risk of his life above and beyond the call of duty while serving as a Rocket Gunner attached to Company F, Second Battalion, First Marines, First Marine Division (Reinforced), in action against enemy aggressor forces near Sosa-ri, Korea, on 17 and September 20, 1950. Dug in a hill overlooking the main Seoul highway when six enemy tanks threatened to break through the Battalion position during a pre-dawn attack on September 17, Private First Class MONEGAN promptly moved forward with his bazooka under heavy hostile automatic weapons fire and engaged the lead tank at a range of less than 50 yards. After scoring a direct hit and killing the sole surviving tankman with his carbine as he came through the escape hatch, he boldly fired two more rounds of ammunition at the oncoming tanks, disorganizing the attack and enabling our tank crews to continue blasting with their 90-mm guns. With his own and an adjacent company's position threatened by annihilation when an overwhelming enemy tank-infantry force by-passed the area and proceeded toward the battalion Command Post during the early morning of September 20, he seized his rocket launcher and, in total darkness, charged down the slope of the hill where the tanks had broken through. Quick to act when illuminating shell hit the area, he scored a direct hit on one of the tanks as hostile rifle and automatic weapons fire raked the area at close range. Again exposing himself he fired another round to destroy a second tank and, as the rear tank turned to retreat, stood upright to fire and was fatally struck down by hostile machine-gun fire when another illuminating shell silhouetted him against the sky. Private First Class MONEGAN's daring initiative, gallant fighting spirit and courageous devotion to duty were contributing factors in the success of his company in repelling the enemy and his self-sacrificing efforts throughout sustain and enhance the highest traditions of the United States Naval Service. He gallantly gave his life for his country.

His Medal of Honor was presented to his widow by the Secretary of the Navy, Dan Kimball, on the 8th of February, 1952, just nine months after their son Walter Monegan III was born. He was originally buried in Inchon, Korea, but on the 19th of July, 1951, Walter Carleton Monegan Jr was reinterred in Arlington National Cemetery; section 34, site 4513.