A micromachined electrometer, based on the concept of a variable capacitor, has been designed, modeled, fabricated and tested. The electrometer has an equivalent noise floor of 245e/vHz, which is over two orders of magnitude better than the best line of commercial instruments currently available for room temperature and ambient pressure operation. As with all other capacitive MEMS applications comprising both mechanical and electrical domain subsystems, system-level equivalent circuit simulation cannot be implemented in most commercial circuit simulators for they do not accept in-line equations for a variable capacitor. This paper uses a voltage source representation method creating a nonlinear time-domain variable capacitor model in SPICE. We show that a circuit model can be realized for any arbitrary nonlinear capacitive MEMS element for embedded co-simulation with interface electronics. The results in the case of a variable capacitor based MEMS electrometer are in good agreement with experimental data. This simple technique is proposed as a method to provide the missing link in aiding design optimization of both electro-mechanical structures and sensor electronics within the same simulation interface.