This paper presents the fabrication and characterization of monolayer graphene nanomechanical resonators with electrical readout. The devices exhibit resonances in the MHz range, and their response to changes in mass and temperature is investigated. The resonant frequency is found to strongly depend on the applied gate voltage, which can be modeled using a membrane model that yields the mass density and built-in strain. Adding or removing mass results in changes in both the density and strain, indicating that adsorbates impart tension to the graphene. Cooling the devices increases the frequency, and the shift rate can be used to measure the negative thermal expansion coefficient of graphene. The quality factor increases with decreasing temperature, reaching ~10^4 at 5 K. These studies lay the groundwork for applications such as high-sensitivity mass detectors.This paper presents the fabrication and characterization of monolayer graphene nanomechanical resonators with electrical readout. The devices exhibit resonances in the MHz range, and their response to changes in mass and temperature is investigated. The resonant frequency is found to strongly depend on the applied gate voltage, which can be modeled using a membrane model that yields the mass density and built-in strain. Adding or removing mass results in changes in both the density and strain, indicating that adsorbates impart tension to the graphene. Cooling the devices increases the frequency, and the shift rate can be used to measure the negative thermal expansion coefficient of graphene. The quality factor increases with decreasing temperature, reaching ~10^4 at 5 K. These studies lay the groundwork for applications such as high-sensitivity mass detectors.