<p>Micro- and nanomechanical resonators are emerging as promising platforms for quantum technologies, precision sensors and fundamental science experiments. To utilize these devices for force sensing or quantum optomechanics, they must be brought in close proximity with other systems for functionalization or efficient readout. Improved understanding of the loss mechanisms in nanomechanical resonators, specifically the advent of dissipation dilution, has led to the development of ultracohorent devices with mechanical quality factors exceeding one billion at room temperature, setting their force sensitivities surpassing those of the state-of-the-art atomic force microscopes. Given this new regime of sensitivity, an intriguing question is whether the proximity of other materials hinders mechanical coherence. Here we report a dissipation mechanism that occurs in ultracoherent nanomechanical oscillators caused by the presence of nearby dielectrics. By studying the parameter scaling of the effect, we show that the mechanism is more severe for low-frequency mechanical modes. This is due to dielectric loss within the materials caused by the motion of a resonator carrying static charges. These observations are consistent with the non-contact friction observed in atomic force microscopes. Our findings provide insights into limitations on the integration of ultracoherent nanomechanical resonators and highlight the adverse effects of charged defects in these systems.</p>