The Yakama people were the first inhabitants of the Yakima Valley. In 1805 the Lewis and Clark Expedition came to the area and discovered abundant wildlife and rich soil, prompting the settlement of homesteaders. A Catholic Mission was established in Ahtanum, southwest of present-day Yakima, in 1847. The arrival of settlers and their conflicts with the natives resulted in the Yakama Indian War of 1855. The U.S. Army established Fort Simcoe in 1856 near present-day Toppenish as a response to the uprising. The Yakamas were defeated and relocated to the Yakama Indian Reservation. Yakima County was created in 1865.
Ahtanum is located at 46°33′40″N 120°36′37″W (46.560998, -120.610237). According to the United States Census Bureau, the CDP has a total area of 9.7 square miles (25.1 km²), all of it land.