Fire activity has increased in western US aridland ecosystems due to increased human-caused ignitions and the expansion of flammable exotic grasses. Because many desert plants are not adapted to fire, increased fire activity may have long-lasting ecological impacts on native vegetation and the wildlife that depend on it. Given the heterogeneity across aridland ecosystems, it is important to understand how trends and drivers of fire vary, so management can be customized accordingly. We examined historical trends and quantified the relative importance of and interactions among multiple drivers of fire patterns across five aridland ecoregions in southeastern California from 1970 to 2010. Fire frequency increased across all ecoregions for the first couple decades, and declined or plateaued since the 1990s; but area burned continued to increase in some regions. The relative importance of anthropogenic and biophysical drivers varied across ecoregions, with both direct and indirect influences on fire. Anthropogenic variables were equally important as biophysical variables, but some contributed indirectly, presumably via their influence on annual grass distribution and abundance. Grass burned disproportionately more than other cover types, suggesting that addressing exotics may be the key to fire management and conservation in much of the area.