Thomas opened the scoring for the Colonels (8-16, 4-8 Southland Conference) with an 8-0 run all by himself, knocking down a pair of 3-pointers and foul shots, and Nicholls State eventually increased the lead to 22-6 with 7:51 before halftime.
Nicholls State led by 10 at intermission and held on despite being outscored 38-37 by the Cardinals in the second half.
The Colonels outshot the Cardinals 55.3 percent (26 of 47) to 30.6 percent (19 of 62) from the floor in the win.
Anthony Miles led Lamar (16-9, 7-4) with 15 points and Mike James added 14, while Devon Lamb finished with 13 points and 10 rebounds. The Cardinals came in having won four straight games, beginning with a victory against Nicholls State on Jan. 28.