McMullen was born in 1889 in Rainsboro, Ohio. He had previously served as Supervising Principal at Polk and Sullivan schools before being hired as Superintendent of Loudonville in 1929. Following the hire, he moved to Loudonville and quickly made a name for himself. Among the numerous improvements he was credited with were establishing bus routes to transport students after one room schools were closed in the 1930s, the construction of the auditorium wing on the Loudonville High School (now C.E. Budd School), planning and construction of a new grade school (now R.F. McMullen Elementary), the introduction of a guidance department, and the enlargement of the school libraries. He was admired by both the faculty and the students for his diplomacy, fairness, flexibility, and his sense of humor.

Following his retirement in 1959, he established the R.F. McMullen Athletic Scholarship. He was elected as a State Representative for Ashland County in 1959, and served on a number of state and local boards including as a trustee of Miami University, the Johnny Appleseed Area Boy Scouts (which awarded him the Silver Beaver medal), the Ashland County Automobile Club, and the Loudonville Library. He was awarded the Jaycee Distinguished Service Award and the Distinguished Senior Citizen Award at the Ohio State Fair in 1970.

McMullen passed away in October 1971. "With the death Saturday night of R.F. McMullen, educator, civic leader, and Christian gentleman, the village of Loudonville lost one of its most distinguished and respected citizens. To estimate the influence of Mr. McMullen on the Loudonville Schools, which he served as superintendent from 1929-1959, would be difficult," read the Loudonville Times after his passing.