The "Cobalt silver rush" (Cobalt, Ontario, Canada), which occurred near the early-20th century, led to the establishment of several silver mines in the region, resulting in the surrounding ecosystems becoming contaminated with mine wastes disposed of directly into lake basins. Ibsen Pond, however, was not used to store tailings. Yet, the elevated levels of metal(loid)s in the surface waters indicated that mine wastes may have entered this lake via a connecting stream. Here, we used a paleoecotoxicological approach to investigate: 1) when migration of mine tailings into this lake began; and 2) how it altered aquatic biodiversity across multiple trophic levels. The geochemical proxies tracked the pollution history of Ibsen Pond, with sharp decreases in sedimentary organic content and concomitant increases in the concentrations of several toxic metal(loid)s between ca. 1910 and 1940, matching the time when regional silver mining activities peaked. The calculated probable effects concentration quotients (PEC-Q) for sediments deposited after mining contaminants entered the lake exceeded the probable biological effects threshold (PEC-Q > 2). Examination of sedimentary diatom (Bacillariophyceae) and cladoceran (Branchiopoda) assemblages revealed notable decreases in the relative abundances of several littoral taxa, along with declines in Hill's N2 diversity across both bioindicator groups. Our paleoecotoxicological analyses show that migration of mine wastes via stream inflow can be an important source of pollution to previously uncontaminated lakes near abandoned mines and ultimately cause substantial biological damage.