Astronomer and philanthropist. Living in Dundee, Mills made his fortune as a linen and twine manufacturer. He had been greatly influenced by Rev. Thomas Dick (1774 - 1857), a clergyman and philosopher who believed the greatness of God could best be appreciated by the study of astronomy.
Mills had the time and resources to become a keen amateur astronomer and built his own private observatory on the slopes of Dundee Law, although this no longer exists.
On his death, he left his estate for the purpose of erecting a public observatory "equipped with astronomical and other instruments suitable for the study of the wonder and beauty of the works of God in creation". After much discussion and many delays, the Mills Observatory on Dundee's Balgay Hill eventually opened in 1935.