The double overtime contest was the third of the season for the Demons (4-8, 0-1) who haven't won one. The Bearkats (9-4, 1-0) got 21 points by Jabari Peters, five in the second overtime when Sam Houston opened a seven-point advantage and kept NSU from getting any closer than the final three-point margin.
Sam Houston sank its final eight free throws in the closing 2:36 of the second OT after a miserable showing at the line (24-44) to that point. Will Bond made four and Peters, who was 9 of 18 at the line, added two along with Mason to abate NSU's comeback.
Michael Holyfield added 20 points for the Bearkats, who got 17, all in the first half, by Paul Baxter as Sam Houston ran to a 47-38 advantage.
Despite sitting out the last 12 minutes of the opening half after picking up two fouls, Jalan West scored 22 points and had six rebounds for the Demons. DeQuan Hicks had 21 points and 13 rebounds, and sank two pressure-packed free throws with 2.5 seconds to go in regulation to tie the game at 83 and force overtime.
Sabri Thompson scored a season-best 16 points, making 4-of-6 on 3-pointers, for the Demons, who got 13 points by Brison White and 12 from Zeek Woodley.
The Demons trailed by as many as 14 points, 59-45, early after halftime before mounting an 18-6 run within 64-63.
They took their first lead at 73-72 on two West free throws, but didn't go on top again until the first overtime.
NSU was up 89-96 when Sam Houston's Darkari Henderson got his only points of the night, on a 3-pointer, to tie the game.
Henderson had a crucial steal with 34 seconds left to keep Northwestern from padding its two-point lead, before Mason's free throws knotted the score with six ticks remaining.
NSU was called for an offensive foul with two seconds to go, negating the Demons' bid to escape with the win after one OT.
NSU will play host to Lamar Saturday afternoon in another Southland doubleheader, with the women's game at 1 and the men following at 3.