The low-mass companion to the nearby subdwarf Mu Cas has been detected by speckle interferometry at 2.2 and 3.4 microns, yielding a major improvement in measurements of the angular separation and apparent magnitudes. Relative to the primary star, Mu Cas B is 2.5 mag brighter at these wavelengths than at 0.55 micron. At a mean epoch of 1983.20, the companion is separated from the primary by 0.98 + or - 0.024 arcsec at position angle 55.8 + or - 0.7 deg. These measurements, together with two independent sets of astrometric measurements, provide the means for mass determinations. One solution yields individual masses (0.89 + or - 0.26 and 0.19 + or - 0.046 solar mass) consistent with the measured absolute magnitudes and with standard main-sequence models. The other solution yields lower masses and requires overluminosities of 1.3-2.5 mag relative to the main sequence. Attempts to determine the primordial helium abundance, Y, from the mass Mu Cas A yield Y = 0.25 + or - 0.25 and are now limited primarily by inaccuracies in the photocentric orbital elements.