The Cowboys (5-11, 3-1 Southland Conference) jumped to a 26-9 lead as Keelan Garrett sank a pair of 3-pointers. Guidry hit two layups and Austin Lewis had a dunk in between to give McNeese State its largest lead, 33-14 with 4:11 remaining in the first half.
Lamar (1-15, 0-4) cut their deficit to nine in the second half after a three-point play by Rhon Mitchell with 8:57 to play, but couldn't pull any closer.
McNeese State shot 45.9 percent from the field and hit eight 3-pointers. Kevin Hardy had 11 rebounds, eight points and six assists for the Cowboys.
Lamar dropped its 12th straight game, shooting 33.9 percent and going 0 of 8 beyond the arc. Mitchell led the Cardinals with 18 points and Amos Wilson had 12.