Marcus was born in Montreal and educated at McGill University there. After graduating he taught at the Polytechnic Institute, Brooklyn (1951–64) and at the University of Illinois (1968–78). In 1978 he was appointed professor of chemistry at the California Institute of Technology.

In the 1950s Marcus began to work on electron-transfer reactions. The addition, removal, and transfer of electrons is the driving force behind many basic chemical processes including photosynthesis, respiration, and the production of solar energy. Such reactions are, in principle, very simple involving the movement of an electron from one ion to form another ion. The rates of such reactions can, however, vary widely. Marcus was able to explain electron-transfer reaction rates in terms of the way in which the solvent molecules, initially configured to solvate the reactant, reorganize to solvate the products.

Marcus was awarded the 1992 Nobel Prize for chemistry for his work in this field.