We report on a direct measurement of the approximate to 8.4 eV nuclear excitation energy of the isomeric first-excited state 229mTh via the internal conversion (IC) decay channel. Thermalized and mass-filtered recoiling 229mTh ions from 233U alpha decay are delivered to the surface of a superconducting nanowire sensor and become embedded. The ion is neutralized, triggering the IC decay, and the energy released by the IC decay is detected with high quantum efficiency by the nanowire sensor. Energy resolution is enabled by the current dependence of the internal quantum efficiency of the nanowire sensor. The techniques presented here are complementary to light-based detection schemes. The IC decay channel is about eight orders of magnitude faster than the photoemission channel, thus the ability to detect IC decays with high efficiency with superconducting nanowire sensors is likely to be a valuable tool for future 229mTh experiments.