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

He was admitted to the bar the same year and commenced practice in Salt Lake City, Utah.

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 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.