Mycorrhizal and saprotrophic (SAP) fungi are essential to terrestrial element cycling due to their uptake of mineral nutrients and decomposition of detritus. Linking these ecological roles to specific fungi is necessary to improve our understanding of global nutrient cycling, fungal ecophysiology, and forest ecology. Using discriminant analyses of nitrogen (δ15N) and carbon (δ13C) isotope values from 813 fungi across 23 sites, we verified collector-based categorizations as either ectomycorrhizal (ECM) or SAP in > 91% of the fungi, and provided probabilistic assignments for an additional 27 fungi of unknown ecological role. As sites ranged from boreal tundra to tropical rainforest, we were able to show that fungal δ13C (26 sites) and δ15N (32 sites) values could be predicted by climate or latitude as previously shown in plant and soil analyses. Fungal δ13C values are likely reflecting differences in C-source between ECM and SAP fungi, whereas 15N enrichment of ECM fungi relative to SAP fungi suggests that ECM fungi are consistently delivering 15N depleted N to host trees across a range of ecosystem types.