Reclaimed coal mine sites represent a potential terrestrial carbon (C) sink. However, there is currently no cost-effective, rapid, and reliable method to quantify soil organic matter accumulation in soils containing appreciable quantities of coal and carbonate media. The derivative thermogravimetry method was evaluated as a potential analytical tool for differentiating C in spoils that may contain both recently derived ("new") and ancient ("old") C fractions. Grass litter and coal were used to represent "new" and "old" organic C, respectively, and limestone to represent the carbonate fraction. Analysis was carried out using a temperature ramp of 20 °C min−1 and a nitrogen flow rate of 20 mL min−1 on a TG analyzer. Derivative thermogravimetry curves showed pyrolysis peaks at distinctively different temperatures: grass litter, 270 to 395 °C; coal, 415 to 520 °C; limestone, 700 to 785 °C. Recoveries from mixtures of these three components at the 95% confidence interval were found to be 94.49% ± 4.23% (coal), 93.67% ± 2.11% (litter), and 108.88% ± 2.88% (limestone). Petrographic analysis was used to validate derivative thermogravimetry findings. Duplicate point count analyses (300 points) of a 1:1:1 mixture of grass litter, coal, and limestone yielded 39.2% ± 1.2% of "new" organic C, 35.0% ± 2.4% of coal, and 25.9 ± 1.2% of minerals (in this case, limestone). These results indicate that organic petrography can be a useful approximation of "new" organic C, but particle density differences and time constraints might limit it. Thermogravimetry appears to be the superior of the two methods as it proved to be a more cost-effective, rapid, and direct method for differentiating and quantifying C.