Systems and methods for determining whether mechanical fatigue exists in a downhole cable using thermally-induced acoustic waves are disclosed herein. A cable fatigue monitoring system includes a thermal source, one or more light sources, one or more photodetector arrays, and a computing system comprising a processor, a memory, and a cable distortion module. The cable distortion module is operable to generate acoustic waves in a cable using the thermal source, direct light from the one or more light sources toward the cable, detect light from the one or more light sources transmitted past the cable at the one or more photodetector arrays, and determine, based on the detected light transmitted past the cable, whether a change in velocity of the acoustic waves has occurred in the cable.