The Golden Eagles (4-3, 2-1 in Conference USA) shrugged off a slow start, in which they led 10-6 at the half, and scored 20 points in the third quarter.
Lampley's punt return, the first for any Southern Miss player since 2003, was the first score of the second half.
Golden Eagles tailbacks Damion Fletcher and Tory Harrison combined for 122 yards on 30 carries, and Fletcher scored twice.
Southern Miss had plenty of help from Memphis (2-5, 1-3), which committed 11 penalties for 105 yards and had two turnovers.
Southern Miss dedicated the game to punter Peter Wilkes, who died of a self-inflicted gunshot wound on last Saturday in Union City, Tenn.