He raised three sons with his wife Betty Coffin, whom he married in 1942.
They lived in Farmington, Iowa, and from 1970, in West Des Moines.
Millen's political experience before election as a state representative included thirteen years with the Farmington Independent School District as president and director.
At the start of his third term, Millen, a Republican, was elected majority leader.
Millen died on 18 May 1998 at Iowa Methodist Medical Center in Des Moines, where he was seeking treatment for cancer.