Milton had 138 yards, Ross totaled 126 and Sams finished with 104 as McNeese State (10-0, 9-0 Southland Conference) had 406 yards rushing compared to Lamar's 63.
Kade Harrington, who had 2,022 rushing yards this season prior to this game, was held to just 70 yards for Lamar (5-6, 4-5).
The Southland Conference champion Cowboys became only the ninth team in the league's 52 years of football to finish the regular season undefeated. Their first game this season, against LSU, was cancelled because of inclement weather.
All the scoring happened in the first quarter until Jean Breaux booted a 31-yard field goal in the fourth and Lamar's Joe Minden hit Devonn Brown on a 32-yard scoring strike to close to 20-14 with 4:42 left in the game.