A test method is for determining total isotropic sensitivity (TIS) of a mobile wireless communications device using an RF source controllable to a plurality of source values having a relatively small uncertainty associated therewith. The mobile wireless communications device may includes an antenna and a RF receiver coupled to the antenna and outputting an RSSI value having a relatively large uncertainty associated therewith. The method may include measuring a sensitivity of the RF receiver, and measuring an antenna gain pattern for the antenna based upon controlling the RF source to respective source values causing the RE receiver to generate a same target RSSI value as the RF source and wireless communications device are angularly moved relative to one another to thereby reduce the relatively large uncertainty of the RSSI value. The TIS may be determined based upon the measured sensitivity of the RF receiver and the measured antenna gain pattern.