A mechanically tunable magnetron injection gun (MIG) provides an annular, relativistic beam of electrons for injection into an axially aligned magnetic field of a gyrotron-class device. The electron emitter encircles a center electrode. Turning a knob adjusts the center electrode's axial position relative to the rest of the cathode. The adjustable center electrode provides an effective means for local field adjustment. The center electrode is located in a particularly sensitive electric field region and adjusts the electric field so as to tune the electron beam from the inside out. Adjusting the center electrode position while the device is in operation is a means for providing mechanical tunability (with respect to beam quality and transverse-to-axial velocity ratio) for the MIG. The mechanical tunability feature provides the MIG with an extra degree of freedom for the optimization of the beam quality, it provides the versatility of operation in a much greater parameter space, it can be used to compensate for machining errors and thermal deformations, and it can provide tunability for single anode MIGs.