In September 2019, destruction of a Vespa mandarinia Smith 1852 nest was reported for the first time in North America in Nanaimo, British Columbia, Canada. In December 2019, the Washington State Department of Agriculture also confirmed the first detection of an adult specimen of V. mandarinia in the United States, in Whatcom County, Washington. Vespa mandarinia is the largest hornet species and is a known predator of several insects, including the European honey bee (Apis mellifera) (Hymenoptera: Apidae) (Linnaeus, 1758). The establishment of V. mandarinia in North America poses a serious threat to apiculture, and this species is considered an actionable quarantine pest. Here we report details of the first detection of this species in the United States and use genetic sequence data obtained from five specimens across the globe to estimate differences in origin of the Canadian and U.S. detections. The full mitochondrial genomes of four V. mandarinia specimens representing different geographic locations were sequenced and compared with an existing reference genome. A maximum likelihood tree using 13 protein-coding regions from mitochondrial DNA suggests that the Canada and U.S. specimens are from two separate maternal lineages. A large-scale survey is currently underway to assess the level of Asian giant hornet establishment in both countries and to determine the future direction of eradication efforts.