Opal Dellinger was born on October 6, 1915 in Rockwell City, Iowa. Dellinger graduated from Rockwell City High School in 1933. She then studied elementary education and music at the Iowa State Teacher's College (now the University of Northern Iowa) for three years. Dellinger married Raburn Gray "Hap" Miller of Zearing, Iowa in 1935. The Millers had six children. "Hap" Miller was a college coach, and the family moved several times as he took jobs at different schools. During this time, Opal Miller served as Deputy Recorder of Warren and Allamakee counties. Upon "Hap" Miller's retirement the couple moved back to Rockwell City.

In 1972 "Hap" Miller was elected to represent District 47 in the Iowa House of Representatives. In April of 1974, before the General Assembly had ended the session, "Hap" Miller died suddenly. Opal Miller ran successfully for his seat in the election of 1974. During her two terms as a Democratic Representative in the Iowa House, Miller served on the House Committees of Agriculture, County Government, Education, the Juvenile Justice Interim Committee, and the Social Services Division of the Budget Committee. After running and winning again in 1976, Miller served her last two years in the Iowa Legislature.

In January of 1978 Miller married her brother-in-law of forty-one years, Charles Sergeant. Sadly, Charles Sergeant died in June of 1979. Shortly thereafter, Opal Sergeant changed her name back to Opal Miller. Miller then moved to Sitka, Alaska to do three semesters of volunteer secretarial work with Volunteers In Mission (VIM), a Presbyterian group. Miller worked at Sheldon Jackson College in Sitka, Alaska, for one and a half years. She then volunteered for three years as secretary at a hospital in Katmandu, Nepal; spent thirteen months in Rutland, Massachusetts with the Heifer Project International (an organization that sent animals to underdeveloped countries), and volunteered at the Menaul School in Albuquerque, New Mexico.

After four years of work at the Menaul School, Miller moved to Des Moines, Iowa. Miller has been involved with the Legislative Ladies' League, Eastern Star, Business and Professional Women's Club, Iowa Women's Political Caucus, National Organization of Democratic Women State Legislators, Calhoun County Historical Society, Federated Women's Club, and the Board of Northwest Iowa Presbyterial. Opal Miller passed away in 2010.