Thornton, the third-round co-leader along with van der Walt and Baptiste Chapellan, emerged with the extra-hole victory after posting a 1-under 70 during his final round.
Van der Walt also notched a 70 Sunday at Aa Saint Omer Golf Club -- both golfers finished 72 holes at 5-under-par 279.
Chapellan, meanwhile, fell out of contention in the final round, stumbling to a 6-over 77. He ended up in a tie for 15th place at 2-over 286.
Sole possession of third place at minus-4 went to Steve Benson (70).