Electro-mechanically tunable meta-surfaces are presented for high gain, beam steerable Leaky-Wave Antennas (LWAs) at 37 GHz and 280 GHz bands. The proposed metasurfaces are: a tunable High Impedance Surface (HIS) in case of 37 GHz LWA, and tunable Partially Reflective Surface (PRS) in case of 280 GHz LWA. The proposed metasurfaces serve as a phase shifter in the beam steering antenna. The required phase shift is achieved by varying the mechanical separation between the HIS/PRS periodic array and ground layer using a piezoelectric actuator (PEA). The presented phase shifting technique offers an extremely low loss solution for antenna beam steering at mm-wave frequencies. The designed antenna at the selected frequency bands may find applications in broadband mobile communications in 5G and beyond. The presented antenna yields a wide S11 bandwidth (BW), high gain and wide beam scanning range as required for broadband mobile applications.