Erosion of soil carbon (C) and nitrogen (N) following severe wildfire may have deleterious effects on downstream resources and ecosystem recovery. Although C and N losses in combustion and runoff have been studied extensively, soil C and N transported by post-fire erosion has rarely been quantified in burned landscapes. To better understand the magnitude and temporal pattern of these losses, we analysed the C and N content of sediment collected in severely burned hillslopes and catchments across the western USA over the first 4 post-fire years. We also compared soil C and N losses from areas receiving common erosion-mitigation treatments and untreated, burned areas. The concentrations of C and N in the eroded material (0.23–0.98 g C kg−1 and 0.01–0.04 g N kg−1) were similar to those of mineral soils rather than organic soil horizons or combusted vegetation. Losses of eroded soil C and N were highly variable across sites, and were highest the first 2 years after fire. Cumulative erosional losses from untreated, burned areas ranged from 73 to 2253 kg C ha−1 and from 3.3 to 110 kg N ha−1 over 4 post-fire years. Post-fire erosion-mitigation treatments reduced C and N losses by up to 75% compared with untreated areas. Losses in post-fire erosion are estimated to be <10% of the total soil C and N combusted during severe wildfire and <10% of post-fire soil C and N stocks remaining in the upper 20 cm of mineral soil. Although loss of soil C and N in post-fire erosion is unlikely to impair the productivity of recovering vegetation, export of C and N may influence downstream water quality and aquatic ecosystems.