Mott polarimeters are used in many areas of physics to determine the spin polarization of electrons in studies of processes dependent on the electron's intrinsic spin angular momentum and associated spin magnetic moment. Mott polarimeters work by measuring the left-right asymmetry in the spatial distribution of electrons scattered from high-Z nuclei. The polarization of the incident electrons is related to the measured asymmetry via a parameter known as the effective asymmetry (Sherman) function, S\sbeff, which must be determined through calibration. This work describes a novel technique for calibrating a Mott polarimeter that makes use of electrons of accurately known polarization obtained through Penning ionization reactions involving electron spin polarized He(2\sp3S) metastable atoms. This technique has been used to calibrate a compact retarding-potential Mott polarimeter, and the values of S\sbeff are presented for both gold and thorium target foils under a variety of operating conditions.