Hybridisation among taxa with different ploidy levels is often associated with hybrid sterility. Clonal reproduction can stabilise these hybrids, but pervasive clonality may have a profound impact on the distribution of genetic diversity in natural populations. Here we investigate a widespread triploid taxon resulting from hybridisation between diploid Mimulus guttatus and tetraploid Mimulus luteus, two species that were introduced into the United Kingdom (UK) in the nineteenth century. This hybrid, Mimulus x robertsii, is largely sterile but capable of prolific vegetative propagation and has been recorded in the wild since 1872. We surveyed 40 Mimulus populations from localities across the UK to examine the current incidence of hybrids, and selected seventeen populations for genetic analysis using codominant markers. Cluster analyses revealed two main groups of genetically distinct individuals, corresponding to either diploid (M. guttatus) or polyploid (M. luteus and M. x robertsii) samples. Triploid hybrids were found in around 50% of sampled sites, sometimes coexisting with one of the parental species (M. guttatus). The other parent, M. luteus, was restricted to a single locality. Individual populations of M. x robertsii were genetically variable, containing multiple, highly heterozygous clones, with the majority of genetic variation distributed among- rather than within populations. Our findings demonstrate that this largely sterile, clonal taxon can preserve non-negligible amounts of genetic variation. The presence of genetically variable hybrid populations may provide the material for the continued success of asexual taxa in diverse environments.