Duncan began his teaching career in the Butte County Public Schools in Arco, Idaho, in 1954. He subsequently became a principal and later the superintendent of schools. Before serving as superintendent, he was elected to the state legislature, serving from 1962 to 1965. He then completed his Ph.D. at the University of Oregon and after a short interim on the faculty at the University became the Superintendent of Schools of the Clackamas County Intermediate Education District. In 1974, he was elected State Superintendent of Schools. After his four-term tenure as Oregon's Superintendent of Public Instruction, Duncan returned to teaching in 1990, serving as dean of the University of Portland’s School of Education until 1996, when he took emeritus professor status. He was appointed the following year by Governor Kitzhaber to fill a vacancy in the Oregon State Senate, and was elected to serve a full four-year term in 1998 after defeating Monroe Sweetland in the November election.