The evolutionary consequences of natural introgression provide a rare opportunity to retrospectively evaluate how the introduction of exotics or genetic rescue efforts may impact endemic faunas. Phylogeographic structure among mainland, endemic insular, and introduced North American marten (Martes americana and M. caurina) populations have been shaped by a complex history of natural, post-glacial population expansion followed by a series of anthropogenic introductions. In some cases, both natural colonization and translocations facilitated secondary contact, offering a series of replicated experiments that demonstrate how introgression, in these cases following isolation (insular and refugial), shapes genetic diversity. We test whether genetic exchange is occurring between North American marten species using mitochondrial genomes and ten nuclear loci. We present evidence of biased nuclear introgression from M. caurina into M. americana across two natural hybrid zones (insular and mainland) and found no remnant evidence of M. caurina on islands that received M. americana translocations, suggesting prior absence, potential extirpation, or genetic swamping of M. caurina from these islands. Our results highlight the importance of understanding phylogeographic variation prior to identifying source populations for wildlife translocations and caution the use of genetic rescue for North American marten populations. Although previously managed as a single species, these two species show substantial genetic divergence. When the two are placed into contact, they exhibit unidirectional, asymmetric introgression with potentially negative consequences for M. caurina, underscoring the value of mindful consideration of introgression in wildlife management.