His first job was with his family's Mills Novelty Company in Chicago, in the summer 1950, working the lowest position at the factory, hauling compressors and returned machinery.
They had two daughters, Natalie (Bontumasi) and Brett, and a son, Julian, as well as two grandchildren, Lucian and Celeste, at the time of his death.
A teacher and critic known for the precision of his observations and the generosity of his praise, Mills first taught at University of Chicago (1959–65), and was Associate Chairman of the Committee on Social Thought.
He had the ability to break down complex poems and "didn't demand his students be carbon copies of who he was", said Michael Anania, a poet and colleague of Mills' at UIC.
Mills committed himself to poetry scholarship early in his career; he edited the letters of the great American poet Theodore Roethke and the notebooks of David Ignatow.
Mills' scholarly essays and reviews have appeared in the most distinguished literary and academic journals, but he has also written with great verve and clarity for a popular audience in the Chicago Sun-Times.
Much of his poetry was in the objectivist style, "dependent on images, tersely presented," said fellow poet Michael Anania.
He worked in a third-floor den of his home, writing in longhand and revising frequently before moving to a typewriter.