 Castillo was the first Hispanic woman be seated in the Oregon Legislative Assembly, and would later become the first to gain election to a statewide office.She was first appointed by the Lane County commissioners to fill a vacancy in the Oregon State Senate in 1996, winning re-election in 1998.She became vice-chair of the Education Committee, dealing with such issues as charter schools, teacher tenure and school reform, and was selected an assistant Democratic leader for the 1999 and 2001 legislative sessions.