While at Notre Dame, Millner was involved in many notable plays. In 1933, Notre Dame was playing unbeaten Army and trailed 12-6 with one minute to play. Then Millner blocked an Army punt and recovered it for a touchdown and Notre Dame won 13-12.
In 1935, both Notre Dame and Ohio State University were unbeaten. Notre Dame trailed 13-0, but then scored two late touchdowns. Millner then caught a touchdown pass from Bill Shakespeare in the closing seconds to beat Ohio State, 18-13 and stayed undefeated.
In 1968, Millner was enshrined in the Pro Football Hall of Fame, becoming only the third Notre Dame player (to that point) to earn the honor. A touching moment took place during the induction ceremonies, when the large crowd there to cheer for former teammate Cliff Battles, also chose to give Millner a huge ovation. Millner had only been accompanied by his wife, Sara.
After having served as a player-coach in 1945, Millner became a full-time assistant for the team the following year, spending three seasons in that role. In 1949, he moved to the All-America Football Conference as a Chicago Hornets assistant, then spent the next year in the same role with the original Baltimore Colts.
In 1951, he was hired as an assistant with the Philadelphia Eagles under Bo McMillin, but when McMillin was diagnosed with terminal stomach cancer, Millner was elevated to interim head coach. He resigned prior to the start of the 1952 NFL season, citing health problems. However, just 17 days after his decision, he accepted an assistant coaching position with the Redskins.
Millner stayed as an assistant until 1957, when he accepted an assistant coaching position at Hardin-Simmons University, working under his old cohort, Baugh. After just one season, Millner resigned the post and worked as a car salesman until returning to the Redskins as a scout in 1963, the same year he suffered his first heart attack.