Seibring's shot put the Phoenix up 72-71 and started a 10-3 game-ending run for Elon (3-2). Mount St. Mary's (0-5) had erased a 10-point Elon lead in the second half and built a seven-point advantage with 7:27 to go.
A pair of free throws by Elon's Luke Eddy tied the game at 69. BK Ashe's dunk gave the Mountaineers back the lead until Seibring's 3-pointer just over a minute later.
Eddy finished with 17 points, nine from the free-throw line, with Steven Santa Ana scoring 13 points, 10 in the first half when the Phoenix took a 38-29 lead. Dainan Swoope added 11 points and Brian Dawkins 10.
Ashe scored a career-high 25 points for Mount St. Mary's. Junior Robinson added 19 and Taylor Danaher 15.