Over the past century, the semi-arid grasslands of southeastern Arizona have experienced changes in vegetation, the most significant of which is encroachment of woody plants. Management of these grasslands has also shifted, with much of the land overgrazed in the early part of the century and more recent implementation of alternate grazing strategies designed to improve rangeland productivity. Taken together, woody encroachment and changing management can affect the stocks of soil carbon. Here we simulate 100 years of vegetation and management change on the Santa Rita Experimental Range and the Appleton-Whittell Research Ranch. Information from historical grazing records, ecosystem research, repeat photography and local experts was gathered and used to simulate net primary productivity and soil carbon dynamics with the EPIC model. We find EPIC capable of reproducing important aspects of this ecosystem, including the impact of different grazing intensities on stocks and dynamics of ecosystem carbon. Both the encroachment of velvet mesquite (Prosopis velutina) and grazing changes have had significant positive impact on soil carbon stocks over the past 100 years. Our findings have implications for the future management of arid grasslands and their soil carbon stocks.