Multi-isotope analysis (e.g., Sr–Pb–O–H–C–N) of human scalp hair is routinely used in forensic investigations of human remains to constrain the geographic origin of unidentified bodies, and to investigate antemortem mobility patterns. However, while it is known that postmortem processes can affect the preservation of, or even overprint, the biogenic isotopic signatures in hair, the speed and nature of these processes have rarely been studied. This study investigates the effects of decomposition and environment on the H–Pb–Sr isotope compositions of human hair as well as the relationship between structural hair shaft degradation and isotopic signature change over time. Human scalp hair samples from four body donations were collected at different stages throughout gross body decomposition. The willed-donated bodies were placed to decompose outdoors at the Forensic Anthropology Research Facility (FARF) at Texas State University. Hair fibers from two of the donations were examined using scanning electron microscopy (SEM) and high-resolution light microscopy (HRLM). Chemical and microbiological degradation of hair fibers occurred rapidly after placement of the body outdoors. Measurements of scalp hair isotopic composition demonstrated that H–Pb–Sr isotope ratios were altered within days after environmental exposure, presumably by deposition, leaching and/or exchange with the local bioavailable soil, and vapor. The degree of physical hair degradation and changes in H–Pb–Sr isotope composition were not correlated. We conclude that antemortem isotopic H–Pb–Sr isotope ratios are difficult to recover in hairs derived from decomposing whole bodies.