This paper describes the development and experimental characterization of a wall shear stress sensor system designed to be non-invasive in high-temperature, harsh environment applications. The MEMS sensor utilizes differential optical transduction of a floating element structure created using a silicon-on-Pyrex fabrication process. Implementation of a folded- tether floating element structure and high-temperature packaging materials enables continuous operation of the sensor in environments up to 400C. Dynamic calibration of a 50 Pa maximum shear stress system yields a sensor resonance of 3.4 kHz, sensitivity of 57.9 mV/Pa at 1 kHz, a pressure rejection ratio of 82 dB, and a noise floor of 8.7 uV/Hz^1/2, corresponding to a minimum detectable shear stress of 0.15 mPa and a dynamic range of 110 dB. Stability testing of the system for mean shear stress measurement demonstrates a measurement accuracy of +/-0.04% full scale (+/-20 mPa) over a 60 min period.