[2][3] He attempted to enlist in the U.S. armed forces during World War I, but was rated 4-F by the draft board due to flat feet, and instead moved to Washington, D.C. to work for the government in a legal capacity.
[4]Nevertheless, Milliken's nomination passed the Senate, and he served on the Board from 1926 until his resignation in 1931.
After Milliken left government service, he moved to Los Angeles, California, where he entered into private practice, eventually representing celebrities including Tom Mix, Clark Gable, and Jack Benny.
Millikin also lectured at USC Law School in the 1930s and 1940s, helping to establish a tax institute there in 1948.
In 1979, he oversaw the purchase of the Belridge Oil Company and production rights on the South Belridge Oil Field by Royal Dutch Shell for $3.65 billion, which was the largest-ever fiscal transaction in the United States to that date.