We describe the first known established Asian longhorned tick (ALT) (Acari: Ixodidae: Haemaphysalis longicornis Neumann) population in Ohio, USA. In mid-summer 2021, we collected ALTs from an infested pasture in response to an alert that grazing cattle had been infested with ticks, and 3 of them had died. No ALTs were reported following pesticide treatment of the pasture in fall 2021. In the laboratory, we identified 9,287 ticks to species, representing all 3 life stages, as ALTs and tested 100 of the adult females for infectious agents relevant to human and animal health, including Theileria orientalis, a cattle disease agent. Eight field-collected ticks were positive for Anaplasma phagocytophilum (n = 100, 8%); no other infectious agents were detected. Active environmental surveillance showed the return of ALTs in June 2022 despite the tick control efforts in 2021. As ALTs continue to expand their range in the United States, active and passive surveillance studies will be needed to characterize their evolving role in human and animal health.