As a child, Robert Mulliken learned the name and botanical classification of plants and, in general, had an excellent, but selective, memory.

At this time, the United States had just entered World War I, and Mulliken took a position at American University in Washington, D.C., making poison gas under James B. Conant.

After the war, he took a job investigating the effects of zinc oxide and carbon black on rubber, but quickly decided that this was not the kind of chemistry he wanted to pursue.

At Chicago, he had received a grant from the National Research Council (NRC) which had paid for much of his work on isotope separation.

He went to Harvard University to learn spectrographic technique from Frederick A. Saunders and quantum theory from E. C. Kemble.

At the time, he was able to associate with J. Robert Oppenheimer and many future Nobel laureates, including John H. Van Vleck and Harold C. Urey.

In 1925 and 1927, Mulliken traveled to Europe, working with outstanding spectroscopists and quantum theorists such as Erwin Schrödinger, Paul A. M. Dirac, Werner Heisenberg, Louis de Broglie, Max Born, and Walther Bothe (all of whom eventually received Nobel Prizes) and Friedrich Hund, who was at the time Born's assistant.

In 1927 Mulliken worked with Hund and as a result developed his molecular orbital theory, in which electrons are assigned to states that extend over an entire molecule.

In World War II, from 1942 to 1945, Mulliken directed the Information Office for the University of Chicago's Plutonium project.

In 1952 he began to apply quantum mechanics to the analysis of the reaction between Lewis acid and base molecules.

In 1961, he became Distinguished Professor of Physics and Chemistry at Florida State University, and continued in his studies of molecular structure and spectra, ranging from diatomic molecules to large complex aggregates.

At the age of 90, Mulliken died of congestive heart failure at his daughter's home in Arlington County, Virginia on October 31, 1986.