Claude Mathis of Texas State held the old Southland Conference single-game mark of 310, set in 1996.
Harrington, who broke for an 80-yard touchdown run in the second quarter, averaged 12.4 yards a carry. He also had a 62-yard run for a score in the fourth quarter.
Harrington is now the career scoring leader at Lamar with 200 points, passing Sammy Carpenter, who had 198 points (1951-54). Harrington also moved to third in career rushing with 2,453 yards.
Joe Minden threw for 130 yards and three touchdowns for Lamar (3-2, 2-1).
Parker McKenzie led Abilene Christian (2-3, 2-2), throwing for 316 yards and three touchdowns.