Mills has been married three times. The first time was from 1961 to 1964, to Russell Alquist, Jr., with whom she had a son, Sean. Her second marriage was from 1975 to 1980 to Michael Miklenda, with whom she had a second child, a daughter, Melissa. In 1980, Mills married Maxwell Caulfield, 18 years her junior. Mills said of the age difference, "Everybody is always interested in the fact that I am married to someone who is a lot younger than I am ... There are no rules, and that's what I believe, because age doesn't really matter. If you meet someone that you're really close to, someone that you love, stick with that."