Millikan graduated from Oberlin College (Oberlin, Ohio) in 1891 and obtained his doctorate at Columbia University in 1895. In 1896 he became an assistant at the University of Chicago, where he became a full professor in 1910.

In 1909 Millikan began a series of experiments (Millikan oil-drop experiment) to determine the electric charge carried by a single electron. He began by measuring the course of charged water droplets in an electrical field. The results suggested that the charge on the droplets is a multiple of the elementary electric charge, but the experiment was not accurate enough to be convincing. He obtained more precise results in 1910 with his famous oil-drop experiment (Millikan oil-drop experiment) in which he replaced water (which tended to evaporate too quickly) with oil.

In 1916 he took up with similar skill the experimental verification of the equation introduced by Albert Einstein in 1905 to describe the photoelectric effect. He used this same research to obtain an exact value of Planck's constant.

In 1921 Millikan left the University of Chicago to become director of the Norman Bridge Laboratory of Physics at the California Institute of Technology (Caltech) in Pasadena, Calif. There he undertook a major study of the radiation that the physicist Victor Hess had detected coming from outer space. Millikan proved that this radiation is indeed of extraterrestrial origin, and he named it “cosmic rays.” As chairman of the executive council of Caltech from 1921 until his retirement in 1945, Millikan turned that school into one of the leading research institutions in the United States.