Samford (16-15), a first-year SoCon member picked to finish sixth in the North Division, will play Chattanooga in Sunday's semifinals.
Montgomery scored seven consecutive points early in the second half, capped by a three-point play with 16:43 remaining, as Samford pulled away to a 44-32 advantage. Samford, which shot 56.4 percent from the floor (22 of 39), stretched its lead to 49-34 on a Josh Davis 3-pointer with 14:57 left.
The Citadel (20-12), hoping to reach the league semifinals for the first time since 1998, was eliminated in its opening game for the sixth time in seven years.
Cameron Wells led The Citadel with 22 points on 9-of-18 shooting.