James spent almost his entire academic career at Harvard. He was appointed instructor in physiology for the spring 1873 term, instructor in anatomy and physiology in 1873, assistant professor of psychology in 1876, assistant professor of philosophy in 1881, full professor in 1885, endowed chair in psychology in 1889, return to philosophy in 1897, and emeritus professor of philosophy in 1907.
James studied medicine, physiology, and biology, and began to teach in those subjects, but was drawn to the scientific study of the human mind at a time when psychology was constituting itself as a science. James's acquaintance with the work of figures like Hermann Helmholtz in Germany and Pierre Janet in France facilitated his introduction of courses in scientific psychology at Harvard University. He taught his first experimental psychology course at Harvard in the 1875-1876 academic year.
During his Harvard years, James joined in philosophical discussions with Charles Peirce, Oliver Wendell Holmes, and Chauncey Wright that evolved into a lively group informally known as The Metaphysical Club in 1872. Louis Menand speculates that the Club provided a foundation for American intellectual thought for decades to come. It was during this time he also had the pleasure of teaching a young Theodore Roosevelt.
James is one of the two namesakes of the James-Lange theory of emotion, which he formulated independently of Carl Lange in the 1880s. The theory holds that emotion is the mind's perception of physiological conditions that result from some stimulus. In James's oft-cited example; it is not that we see a bear, fear it, and run. We see a bear and run, consequently we fear the bear. Our mind's perception of the higher adrenaline level, heartbeat, etc., is the emotion.