The breakdown phase of the startup of metal halide lamps is typically through a cold fill of a rare gas and the ambient vapour pressure of a dose of metals. The dynamics of the breakdown stage are of interest for improving the efficiency and lifetime of lamps. A computational investigation of the breakdown of Ar/Xe mixtures in an idealized lamp geometry was performed using global and two-dimensional (2-d) models to provide insight into the lamp ignition processes and to facilitate comparison with experiments. The experimental trends for breakdown for pressures of 10–90 Torr were qualitatively captured with the global model. Quantitative agreement required accounting for the temporal and spatial plasma dynamics included in the 2-d model. Small fractions of Xe in Ar were found to decrease the breakdown time as the ionization rates increased due to the lower ionization potential of xenon, while the electron energy distribution was not significantly affected. With higher Xe fractions the electron temperature in the ionization front decreased due to there being larger momentum transfer and inelastic losses to the Xe, and as a result the breakdown times increased. The compression of voltage ahead of the ionization front produced large electric fields at the cathode that enabled significant contributions to ionization by secondary electrons.