 In 1983, he joined the Associated Counsel for the Accused (ACA), which provides legal representation for indigent defendants in King County, Washington.Prothero moved into the felony unit of ACA in 1987, and ultimately became an expert in the areas of forensic DNA evidence, offender sentencing, and the death penalty.Prothero was co-lead counsel in State v. Ridgway, the Green River Killer, the largest serial murder case in United States history.