The Catamounts (11-8, 4-0 Southern) pushed ahead in the second half with Sinclair hitting a 3-pointer and Sumler converting a four-point play to spark a 10-1 run and put Western Carolina up 51-41.
Trailing 62-52, Samford (7-13, 1-4) surged back with an 11-2 run capped by a Tim Williams layup with 2:57 remaining. After two Sinclair free throws, Williams split a pair from the line to keep the deficit at two.
Samford's Raijon Kelly couldn't hit a 3-pointer with four seconds left and Western Carolina held on despite finishing 1 of 6 from the free-throw line in the final minute.
Samford had a slight advantage in shooting percentage (43.8 percent) but was outrebounded 38-30. Brandon Boggs had 10 points for the Catamounts.
Isaiah Williams led the Bulldogs with 15 points.