We demonstrate active control of propagating surface waves on a mid-infrared extraordinary optical transmission grating. The surface waves are excited at the interface between a GaAs substrate and a periodically patterned metal film using a dual wavelength quantum cascade laser. The spectral properties of the laser and the transmission grating are characterized by Fourier Transform Infrared spectroscopy. In addition, the far-field emission from excited surface waves at the metal/GaAs interface is studied using a novel spatial and spectral imaging technique. By actively controlling the optical properties of the grating, we demonstrate the ability to control the coupling of incident coherent radiation to surface waves on the grating. With increased tunability of the grating, directional control of excited surface waves should be achievable. These results suggest that the development of actively tunable plasmonic structures could result in plasmonic routers and switches for interconnect or sensing applications.