This study investigates early biological mechanisms underlying the relationship between environmental toxicant exposure and cognitive vulnerability in aging. Using a cross-sectional sample of 126 cognitively unimpaired older adults, we examined associations among non-dioxin-like polychlorinated biphenyls (PCBs), hypothalamic–pituitary–adrenal (HPA) axis function, and subjective cognitive decline (SCD). Higher PCB exposure was associated with a blunted cortisol awakening response increase (CARi) and greater subjective memory complaints, while no significant associations were observed with objective cognitive performance. These findings suggest that stress-sensitive neuroendocrine dysregulation may represent an early pathway linking environmental toxicants to cognitive vulnerability, with SCD potentially serving as an early indicator prior to detectable cognitive impairment.