Increases in soil organic carbon (SOC) during secondary succession in Mediterranean and semi-arid climates, global hot-spots for agricultural land abandonment, have been notoriously difficult to predict and are subject to multiple environmental and land management factors. Field studies have reported positive, negative and no change varying over extended periods of time. To better evaluate the potential carbon sink capacity of regenerating semi-natural landscapes in these climates requires an improved understanding of the rates of SOC gains and losses. We compiled Mediterranean and semi-arid chronosequences and paired plots to investigate the effects of past land use, restoration intensity, and various environmental factors on SOC stocks during post-agricultural succession. Based on a preliminary synthesis of the western Mediterranean basin, we expect significant long-term accumulation rates globally although with high variability and the potential for net losses (compared to cropland control sites) even after several decades. Losses or minimal change are likely due to high initial SOC stock at the time of abandonment (e.g. from anthropogenic organic matter inputs) and too high or too low mean annual precipitation (e.g. < 450 or > 1000 mm), among other factors. A consolidated SOC accumulation rate for both Mediterranean and semi-arid soils undergoing post-agricultural succession is provided to better inform decision-makers on the benefits and challenges of agricultural land abandonment.