The engineering applications of ammonia extend far beyond the pressure and temperature ranges for which thermodynamic models currently exist in the literature. Thus, a thermal non-equilibrium thermochemical model was developed to compute the composition and thermodynamic properties of ammonia for an extended temperature and pressure range that includes ionization reactions. Thermal non-equilibrium between electrons and heavy particles was included and is presented for ratios of 1/2, 1, 2 and 3. The fourteen-equation nonlinear system produced under the assumptions of ideal gas and two-temperature local thermodynamic equilibrium was solved numerically using a Newton-Raphson method. The thermochemical model is verified for both the composition and thermodynamic properties by comparisons to existing thermochemical models in the literature. These comparisons verify the model for the available, yet limited, temperature and density ranges. Analysis of the composition and thermodynamic properties as a function of the independent properties confirms the necessity for such a model as part of rigorous computations with computational fluid dynamics (CFD) or magnetohydrodynamics (MHD) computer codes. The model can be easily cast in tabular form to complement the set of conservation equations utilized by such codes.