Michael McNamara was born in 1841 in County Clare, Ireland, and later immigrated to the United States. He enlisted in the U.S. Marine Corps on November 19, 1868, in Brooklyn, New York, and served aboard the screw sloop USS Benicia during the Korean Expedition in 1871. Private McNamara was awarded the Medal of Honor on February 8, 1872, for his heroic actions aboard the Benicia during the Battle of Ganghwa on June 11, 1871. He was honorably discharged from the Marine Corps on November 19, 1872, and died on April 25, 1907. Michael McNamara was buried at the Holy Name Cemetery in Jersey City, New Jersey.

His Medal of Honor Citation reads:

On board the U.S.S. BENICIA during the capture of the Korean forts, 11 June 1871. Advancing to the parapet, McNamara wrenched the match-lock from the hands of an enemy and killed him.