A key uncertainty concerning the effect of wildfire on carbon dynamics is the rate at which fire‐killed biomass (e.g., dead trees) decays and emits carbon to the atmosphere. We used a ground‐based approach to compute decomposition of forest biomass killed, but not combusted, in the Biscuit Fire of 2002, an exceptionally large wildfire that burned over 200,000 ha of mixed conifer forest in southwestern Oregon, USA. A combination of federal inventory data and supplementary ground measurements afforded the estimation of fire‐caused mortality and subsequent 10 year decomposition for several functionally distinct carbon pools at 180 independent locations in the burn area. Decomposition was highest for fire‐killed leaves and fine roots and lowest for large‐diameter wood. Decomposition rates varied somewhat among tree species and were only 35% lower for trees still standing than for trees fallen at the time of the fire. We estimate a total of 4.7 Tg C was killed but not combusted in the Biscuit Fire, 85% of which remains 10 years after. Biogenic carbon emissions from fire‐killed necromass were estimated to be 1.0, 0.6, and 0.4 Mg C ha−1 yr−1 at 1, 10, and 50 years after the fire, respectively; compared to the one‐time pyrogenic emission of nearly 17 Mg C ha−1.