For many plants, asexual, clonal reproduction appears to be the predominant form of reproduction at their geographic range edge, but how long clonal populations persist and their role in range dynamics are largely unknown. If asexuality enables well-adapted genotypes to persist, this may allow species to expand beyond their sexual niche. Under this scenario, genomic signatures of long-term asexuality should be detectable in range-edge populations. We investigated this hypothesis in the wetland plant, Decodon verticillatus (Lythraceae), which reproduces predominantly through clonal reproduction at its northern range limit. We assembled the transcriptome de novo, identified single nucleotide polymorphisms (SNPs), and compared patterns of genetic variation in sexual and asexual populations at and approaching the range limit. Asexual genotypes exhibited several signatures of long-term asexuality including higher heterozygosity, fewer unique homozygous SNPs (doubletons), and a breakdown of isolation by distance. They also tended to include more deleterious non-synonymous and radical amino acid altering mutations. However, the frequency of unique heterozygous SNPs (singletons) did not differ between sexual and asexual genotypes, and average genetic differentiation was unexpectedly higher among sexual than among asexual genotypes. Yet, overall, our results are consistent with the hypothesis that asexual reproduction enabled D. verticillatus to expand its range further north than would have been possible under sexual reproduction alone. Understanding the factors that influence range dynamics is becoming increasingly important to better anticipate the capacity of species to adapt and shift their ranges in response to anthropogenic environmental changes and to prioritize range-edge populations for conservation.