With Old Dominion playing a man down, Collin Mocyunas scored in the 65th minute and Marshall sent the Monarchs to their first Conference USA loss of the season on Saturday.
Niko Klosterhalfen gave the Monarchs (5-3-1, 3-1) an early lead and then assisted a Daniel Corona goal shortly after halftime.
Jamil Roberts and Illal Osmanu scored within three minutes of each other in the first half for the Thundering Herd (3-4-2, 1-1-1).
Jimmy Filerman was shown a red card in the 39th and the Monarchs played a man down as the Thundering Herd pulled away late.