We report seven successful observations of the astrometric binary GJ 164 AB system with aperture masking interferometry. The companion, with a near-infrared contrast of 5: 1 was detected beyond the formal diffraction limit. Combined with astrometric observations from the literature, these observations fix the parallax of the system, and allow a model-independent mass determination of both components. We find the mass of GJ 164 B to be 0.096 +/- 0.008 M(circle dot). With data taken with the newly commissioned TripleSpec spectrograph, we examine GJ 164's near-infrared spectral features and compare them to a sample of M dwarfs of known metallicity. Models are not consistent with color and mass, requiring a very young age to accommodate a secondary too luminous, a scenario ruled out by the kinematics.