The author's nonequilibrium probability distribution is tested for time-varying mechanical work. Nonequilibrium Monte Carlo (NEMC) is used to simulate a Brownian particle in a soft-sphere solvent, driven by a moving external potential. Results are obtained for the phase lag and amplitude for drive frequencies ranging from the steady state to the transient regime. This now extends the application of the NEMC algorithm to a time-varying nonequilibrium system. The results are shown to agree with those obtained by nonequilibrium stochastic molecular dynamics and by Nosé-Hoover molecular dynamics, from which it is concluded that the nonequilibrium probability distribution correctly describes time-varying mechanical work and that it provides a fundamental basis for nonequilibrium statistical mechanics.