Grassland soil organic carbon (SOC) stock constitutes the largest terrestrial greenhouse gas sink. Therefore, we compared the effect of three common mountain grassland management regimes on the SOC stock in the inner-alpine Lower Engadine valley. We hypothesized that higher management intensity (MI), including higher manure loads and higher cutting frequency, increases plant productivity and organic matter (OM) input to the soil, providing the substrate for a higher SOC stock. In our study, we found that the SOC stock (0–20 cm depth) in low MI plots was only 73.1 ± 3.99 t C ha−1 and much larger (+23%) in medium MI grassland plots with 89.6 ± 4.2 t C ha−1. Surprisingly, in high MI plots, the SOC stock was also comparatively small with 78.7 ± 5.4 t C ha−1 (+8% compared to low MI). Soil pH, C/N ratio, clay content and farmyard manure were no drivers of SOC stock differences. We conclude that, compared to medium MI, in the high MI grasslands, the organic C input from high plant productivity is overcompensated by antagonistic management effects, favoring higher OM decomposition rates. Accordingly, more fertilization and frequent cutting, leading to higher grassland yields, are not necessarily beneficial for SOC sequestration.