He was born in Leatherwood in Lawrence County to William Duncan and Mary (Malott) Duncan, who had come from Kentucky. After fighting in the Civil War, Duncan returned to Bloomington and graduated from Indiana University in 1868 and again from the law school in 1870.
In 1872 he married the former Sadie Cummings, and they had five children, including Frank, Sallie and Newland Duncan, and Mrs. B. E. Hansen. Over his career, he went into partnerships with three local lawyers: Moses Fell Dunn, John Buskirk, and and Ira Batman.
Duncan was elected State Senator from Monroe, Greene and Brown counties in 1894 and was appointed Judge of the Circuit Court of Monroe and Lawrence counties in 1890. He was president of the Workingman's Building and Loan Association, and a director in the Citizens Loan & Trust Company.
Reputedly 6'10" tall, he was also a vice president of the First National Bank.