Optical voltage sensors are widely used for high-voltage (HV) sensing, but the measurement accuracy is often compromised by a sensitivity drift due to variations in the ambient conditions. Synthetic-heterodyne demodulation is a useful technique for dynamic displacement measurements using interferometric sensors as it can provide a detected signal that is immune to an interferometric drift. In this paper, a new synthetic-heterodyne technique, employing the Hilbert transform and gain control feedback, is applied to a polarimetric-based optical sensor system and used to measure an HV signal. The system comprises an He-Ne laser source, optical phase modulator, and HV sensor located between two polarizers followed by a photodetector and real-time electrical signal acquisition and processing. The use of a control loop results in significant improvements in the stability and accuracy of the detected HV signal. The sensor system was used to measure a 60-Hz HV signal with an amplitude range of 300 V to 7.5 kV. The sensor was also used to successfully measure the total power line harmonic distortion of a highly distorted 4 kV signal. The experimental results show very good agreement with measurements obtained by using the industry standard signal coincidence method.