Mercer (16-13, 11-5 Southern) held the lead from the opening points to the final minute. Samford (12-17, 5-11) took its only lead, at 61-60, on Tyler Hood's basket with 26.2 seconds left.
Phillip Leonard put the Bears back in front with 6.9 seconds on a driving layup from the left wing.
Samford's Christen Cunningham then drove the length of the court in the final seconds and fed an open Evan Taylor in the middle of the lane. Taylor drop-stepped before Nwamu met him near the rim for the game-sealing block.
Jibri Bryan added 10 points for Mercer which led by as many as 14.