Diet composition of leaf-eating mangrove crabs is a puzzle among mangrove ecologists. Nutrient-poor leaf litter can in most cases not support animal growth. Food partitioning (mangrove leaves, animal tissue, and microphytobenthos [MPB]) of sesarmid and ucidid mangrove crabs from eight locations in four countries was assessed using the concentration-dependent stable isotope mixing model, IsoConc. While stable C and N isotopes have the potential to track the origin and proportion of food sources for these crabs, only few values of trophic discrimination are available. This problem was addressed here for crabs foraging on leaf litter to identify discrimination values that provide a balanced diet with sufficient nutrients (i.e., N) when combined with other food sources. The data from all mangrove locations suggest that sesarmid and ucidid crabs ingest and assimilate mixtures of available food items. Leaf litter in the form of brown leaves was always the most important C source, while animal tissue in the form live and dead “prey” or MPB in the form of diatoms at the sediment surface were the dominant N sources. Model scenarios were generated to obtain the upper thresholds of trophic 13C discrimination between the examined crab species and leaf litter, while complying with the molar C/N < 20 requirement for sustainable invertebrate nutrition. The model predicts litter 13C discrimination thresholds of +1.5‰ to +6.0‰ for sesarmid crabs and −0.3‰ to +0.6‰ for ucidid crabs. The difference is probably caused by metabolic disparities between these two crab families. Deviations in 15N discrimination have in most cases only minor influence on the model-based 13C discrimination thresholds. The present findings lead us to suggest a modified Optimal Foraging Theory for leaf-eating mangrove crabs.