After being forced to leave Tubac due to the Apache siege, Poston went to Washington D.C. and worked for General Heintzelman as a civilian aide. During this time Heintzelman introduced him to President Abraham Lincoln. Poston in turn used this time to lobby both Lincoln and Congress for creation of an Arizona Territory, advertising the benefits of the area's mineral wealth to the Union cause. Toward the end of the process Poston attended an oyster dinner where the slate of territorial positions was divided among lame duck congressmen and Poston was selected as an Indian Agent for the new territory.