In the age of Internet-of-Things (IoT), 5G wireless networks, the drive towards decarbonization of the energy system, and to reducing environmental pollution, developing a technology for light-harvesting self-powered sensors can significantly contribute to the sustainable development of human civilization. In this paper we propose a light-harvesting self-powered mechanical sensing concept with a simple monolithic structure. We successfully demonstrate the excellent operation of our sensors under different photoexcitation conditions from natural to artificial lights and from weak to strong light powers. Under illumination of a commercial light emitting diode (LED) with a power of 46 µW, the generated lateral photovoltage is of 3.07 mV, which increases by 6, 13, and 20 µV under applied strains of 225, 450, and 675 ppm, respectively. Upon the increase of the light power to 428 µW, the lateral photovoltage reaches 16.42 mV under the free-strain condition, and increases by 11, 25, and 45 µV under the same strain conditions. In addition, even under room light, the lateral photovoltages are 0.86 mV, 0.86 mV + 4.2 µV, 0.86 mV + 8.0 µV, and 0.86 mV + 11.5 µV under strains of 0, 225, 450, and 675 ppm, respectively. Interestingly, the sensitivity of the sensor increases from 0.017 µV/ppm under ambient lighting to 0.03 and 0.065 µV/ppm under LED light powers of 46 µW and 428 µW, respectively. These promising results indicate that sensitivity of the self-powered strain sensor can be readily tuned by controlling illumination. The work offers a promising concept-of-proof methodology for the development of self-powered mechanical sensors.