The effect of nutrient enrichment on mangrove sediment accretion and carbon accumulation rates is poorly understood. Here we quantify sediment accretion through radionuclide tracers to determine organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) accumulation rates during the previous 60 years in both a nutrient-enriched and a pristine mangrove forest within the same geomorphological region of south-eastern Brazil. The forest receiving high nutrient loads has accumulated OC, TN and TP at rates that are 4, 2 and 8 fold higher than those from the undisturbed mangrove. Organic carbon and TN stable isotopes (δ13C and δ15N) signal an increased presence of organic matter (OM) originating with either phytoplankton, benthic algae or another allochthonous source within the more rapidly-accumulated sediments of the impacted mangrove. This suggests that the accumulation rate of OM in eutrophic mangrove systems may be enhanced through the addition of autochthonous and allochthonous non-mangrove material.