Fifteen Guanacos were introduced to Staats Island in Falklands/Malvinas archipelago from Patagonia in the 1930s. After introduction, the Guanaco population increased to almost 400 animals that retained a footprint of the founding effect and bottleneck reflected in the genetic status of this isolated population. The goals of this study were to (i) make a genetic assessment of this island population through comparisons with mainland populations and simulation, and (ii) assess the likely source population of the introduced Guanacos. Genetic variation estimated from 513 bp of mitochondrial DNA sequence and 15 microsatellite loci were compared among 154 Guanacos collected from eight localities, including the adjacent mainland and the islands of Tierra del Fuego and Staats Island. Of the 23 haplotypes observed among our samples, the Staats Island population only contained three haplotypes, all of which were shared with the Monte Leon population in southern Patagonia. Mitochondrial DNA and microsatellite variation on Staats Island were comparable to most mainland populations and greater than those observed on Tierra del Fuego. Patterns of genetic structure suggest that the Staats Island Guanaco population was founded with animals from southern Patagonia (as opposed to northern Patagonia or Tierra del Fuego), but that effective reductions in population size lasted only a few generations and that surviving animals were a random sample of the pre-bottleneck genetic variation.