He received an Artium Baccalaureus degree from Princeton University in 1938 and a Bachelor of Laws from Harvard Law School in 1941, after which he joined the United States Army, serving until 1945.
In 1966, McMillen became a judge of the Circuit Court of Cook County, serving until his appointment to the federal bench.
[1] He presided over the bankruptcy of the Chicago, Milwaukee, St. Paul and Pacific Railroad ("Milwaukee Road") from 1977 until its sale to the Soo Line Railroad in 1986.
McMillen was nominated by President Richard Nixon on March 29, 1971, to a seat vacated by Judge William Joseph Campbell on the United States District Court for the Northern District of Illinois.
McMillen served in that capacity until he retired on September 8, 1985, and returned to private practice.