Lamar (4-5, 3-4 Southland Conference) didn't trail until Condado's kick, which was set up by Sean Eppinette's fumble recovery at the Colonels' 33 with 10:01 left.
Condado missed an extra point following a 5-yard touchdown run by Landry Klann with 5:08 remaining in the third, leaving Nicholls behind 28-27.
The Cardinals used a 13-play, 74-yard drive on their last possession to set up a short field attempt, but missed a 23-yarder as time expired.
Nicholls (2-7, 2-5), which came into the game averaging 162.2 yards rushing, finished with 415. Tobias Lofton set career highs with 17 carries and 154 yards - including an 82-yard touchdown run - and Michael Henry added 100 yards and two touchdowns.
Lamar's Kade Harrington finished with 255 yards rushing, giving him 1,805 this season, breaking the Southland Conference single-season record with two games to play.