Born in Hamilton, Ohio, Millikin graduated from the law school of the University of Colorado at Boulder in 1913.

During World War I he enlisted as a private in the Colorado National Guard in 1917, saw action in France and was mustered out as a lieutenant colonel.

Millikin resumed the practice of law in Denver, Colorado, and became president of Kinney-Coastal Oil.

Millikin was appointed by Governor Ralph Lawrence Carr on December 20, 1941, and subsequently elected on November 3, 1942, as a Republican to the United States Senate to fill the vacancy in the term ending January 3, 1945, caused by the death of Alva B. Adams.

[1] He also voted for an FEPC bill in 1950 in addition to bolstering President Harry Truman's army desegregation.