Dallas Cameron hit three 3-pointers and scored 13 for the Lumberjacks (17-3, 7-0 Southland) and Jared Johnson added 11 points.
Lamar (10-10, 4-4) shot 40 percent from the field in the first half, compared to just 32.5 percent for Stephen F. Austin. The Cardinals were 0 for 6 from 3-point range, made just 3 of 8 free throws and committed 15 turnovers before the break.
Stephen F. Austin capitalized by scoring 17 points off turnovers and seven second-chance points to take a 39-23 advantage at halftime.
Lamar cut its deficit to 62-51 with 9:47 left, but would get no closer as the Lumberjacks shot 52 percent (13 of 25) in the second half.
Anthony Holliday led the Cardinals with 25 points.