We report the first experimental demonstration of a three-terminal nanoelectromechanical field effect transistor (NEMFET) with measurable subthreshold slope as small as 6 mV/dec at room temperature and a switching voltage window of under 2 V. The device operates by modulating drain current through a suspended nanowire channel via an insulated gate electrode, thus eliminating the need for a conducting moving electrode, and yields devices that reliably switch on/off for up to 130 cycles. Radio-frequency measurements have confirmed operation at 125 MHz. Our measurements and simulations suggest that the NEMFET design is scalable toward sub-1 V ultrahigh-frequency operation for future low-power computing systems.