Harrington, who has rushed for at least 100 yards in eight straight games, now has 2,022 rushing yards for the Cardinals (5-5, 4-4) this season.
Harrington broke for a 45-yard scoring run early in the fourth to give Lamar a 21-7 lead.
Incarnate Word (5-5, 4-4) closed to 21-14 when Junior Sessions charged in from the 2 with 14:14 left in the game. Lamar answered quickly when Joe Minden, who threw for 73 yards and one score, hit Martell Hawthorne on a 7-yard scoring pass to give Lamar a 28-14 lead with 8:50 remaining.
Brittain threw for 313 yards.