Many applications based on resonant nanoelectromechanical systems (NEMS) require monitoring their natural frequency of oscillation over time with high precision, e.g. for gas sensing or nanomechanical mass spectrometry. In this study, we integrate a very-high frequency, nanomechanical resonant-body field-effect transistor into a frequency-modulated phase-locked loop (FM-PLL) which operates analog, requires only one frequency source, and simultaneously exploits the low-noise displacement detection based on FM-demodulation with resonant transistors. We demonstrate sustained mechanical oscillation of a 120 MHz doubly-clamped nano-resonator (54 nm thick, 158 nm wide, 2.65 μm long) by using the FM-PLL in vacuum and in air, reaching a frequency stability in the low ppm-range at room temperature.