The differential cross section for ionization of atoms and molecules by proton impact is modeled over a wide range of incident-proton and ejected-electron energies by an analytical equation. This equation is based on the molecular promotion model at low energies and on the classical binary-encounter approximation, modified to agree with the Bethe theory of ionization, at high energies. Three adjustable parameters are sufficient to fit the experimental electron energy distribution at any given impact energy. Equations are given which fit two of these parameters as a function of proton energy over the range of 5 keV to 5 MeV. The third parameter is found to be a constant, independent of proton energy, but its value varies somewhat from target to target. The relation of one of these parameters to the optical oscillator strength is discussed as is the extension of the model to multishell targets. As examples, the model is applied to fit data on molecular hydrogen, helium, and argon. Quantities such as the stopping cross section, the average ejection energy, and the fraction of electrons ejected with energies greater than a given value are easily calculated using this model.