A graduate of Central University of Kentucky, now Centre College, and Louisville Presbyterian Theological Seminary, he was licensed to preach in April 1909 and soon left the country to begin a period of more than thirty years in Hangzhou, China.

He worked as a Presbyterian missionary from 1911 to 1932 before joining the faculty of Hangchow Christian College and eventually becoming the school's president for four years.

After a seven-month detainment in a Japanese prison camp, McMullen returned to the United States in 1943 and was elected president of his alma mater the next year.

[4] After a one-year stint as an assistant principal at Stanford High School in Stanford, Kentucky, he graduated with a Bachelor of Divinity degree from Louisville Presbyterian Theological Seminary in 1909 and later earned a Ph.D. from the Southern Baptist Theological Seminary and two degrees, a Master of Arts and a Doctor of Education, from Columbia University.

[8] After spending seven months in a Japanese prison camp during that country's occupation of China,[8] he returned to the United States in December 1943 aboard MS Gripsholm.

[6] McMullen arrived in Danville on August 24, 1944, and began his term as president of Centre College on September 1, holding the position along with McLeod.

[20] McLeod resigned five days later, effective immediately, leaving McMullen to lead the school as its sole president for the following eleven months.