A curtain wall system having a mullion mounting tongue screw-race configuration for improving the efficiency and reliability of curtain wall elements assembled thereto. The curtain wall system includes a face, a first leg, and a second leg. The first and second leg are integrally formed with and extend outwardly from the face and are interconnected via an intermediate web section. Each of the first leg and the second leg include an outer screw-race lip and an inner screw-race lip. The inner screw-race lips and the outer screw-race lips are disposed on opposite sides of the intermediate web section to form a screw race. The inner screw-race lips and the outer screw-race lips are spaced apart by a distance affording engagement by a threaded fastener against each of the inner screw-race lips and the outer screw-race lips. A thickness of the intermediate web section is less than a thickness of the first leg and of the second leg.