McMillin served as the head football coach at Centenary College of Louisiana (1922–1924), Geneva College (1925–1927), Kansas State University (1928–1933), and Indiana University (1934–1947), compiling a career college football coaching record of 140–77–13. He then returned to the NFL, coaching the Detroit Lions from 1948 to 1950 and the Philadelphia Eagles for the first two games of the 1951 season before he succumbed to a fatal illness. His career NFL coaching mark was 14–24. McMillin's legendary "poor mouthing", pronounced in his distinctive Texas drawl, was in sharp contrast to his teams' successes. He was inducted into the College Football Hall of Fame as a player in 1951.