William James was born at the Astor House in New York City in 1842. He was the son of Henry James Sr., a noted and independently wealthy Swedenborgian theologian well acquainted with the literary and intellectual elites of his day. The intellectual brilliance of the James family milieu and the remarkable epistolary talents of several of its members have made them a subject of continuing interest to historians, biographers, and critics.

William James received an eclectic trans-Atlantic education, developing fluency in both German and French. Education in the James household encouraged cosmopolitanism. The family made two trips to Europe while william James was still a child, setting a pattern that resulted in thirteen more European journeys during his life. His early artistic bent led to an apprenticeship in the studio of william Morris Hunt in Newport, Rhode Island, but he switched in 1861 to scientific studies at the Lawrence Scientific School of Harvard University.

He took up medical studies at Harvard Medical School in 1864 (according to his brother Henry James, the author). He took a break in the spring of 1865 to join naturalist Louis Agassiz on a scientific expedition up the Amazon River, but aborted his trip after eight months, as he suffered bouts of severe seasickness and mild smallpox. His studies were interrupted once again due to illness in April 1867. He traveled to Germany in search of a cure and remained there until November 1868; at that time he was 26 years old. During this period, he began to publish; reviews of his works appeared in literary periodicals such as the North American Review.

James finally earned his M.D. degree in June 1869 but he never practiced Medicine. What he called his "soul-sickness" would only be resolved in 1872, after an extended period of philosophical searching. He married Alice Gibbens in 1878. In 1882 he joined the Theosophical Society.

James investigated mystical experiences throughout his life, leading him to experiment with chloral hydrate (1870), amyl nitrite (1875), nitrous oxide (1882), and peyote (1896). James claimed that it was only when he was under the influence of nitrous oxide that he was able to understand Hegel. He concluded that while the revelations of the mystic hold true, they hold true only for the mystic; for others, they are certainly ideas to be considered, but can hold no claim to truth without personal experience of such. American Philosophy: An Encyclopedia classes him as one of several figures who "took a more pantheist or pandeist approach by rejecting views of God as separate from the world."

During his Harvard years, James joined in philosophical discussions and debates 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 (2001) suggested that this Club provided a foundation for American intellectual thought for decades to come. James joined the Anti-Imperialist League in 1898, in opposition to the United States annexation of the Philippines.

James spent almost all of his 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.

William James was the son of Henry James (Senior) of Albany, and Mary Robertson Walsh. He had four siblings: Henry (the novelist), Garth Wilkinson, Robertson, and Alice. william became engaged to Alice Howe Gibbens on May 10, 1878; they were married on July 10. They had 5 children: Henry (born May 18, 1878), william (born June 17, 1882), Herman (born 1884, died in infancy), Margaret (born March, 1887) and Alexander (the artist) (born December 22, 1890).

One faction sees individuals (as seen in Dickens' A Tale of Two Cities and Thomas Carlyle's The French Revolution, A History) as the motive power of history, and the broader society as the page on which they write their acts. The other sees society as moving according to holistic principles or laws, and sees individuals as its more-or-less willing pawns. In 1880, James waded into this controversy with "Great Men, Great Thoughts, and the Environment," an essay published in the Atlantic Monthly. He took Carlyle's side, but without Carlyle's one-sided emphasis on the political/military sphere, upon heroes as the founders or overthrowers of states and empires.

In 1884 James set the terms for all Future discussions of determinism and compatibilism in the free will debates with his lecture to Harvard Divinity School students published as "The Dilemma of Determinism." In this talk he defined the Common terms "hard determinism" and "soft determinism" (now more commonly called "compatibilism").

James was a founding member and vice President of the American Society for Psychical Research. The lending of his name made Leonora Piper a famous medium. In 1885, the year after the death of his young son, James had his first sitting with Piper at the suggestion of his mother-in-law. James was soon convinced that Piper knew things she could only have discovered by supernatural means. He expressed his belief in Piper by saying, "If you wish to upset the law that all crows are black, it is enough if you prove that one crow is white. My white crow is Mrs. Piper." However, James did not believe that Piper was in contact with spirits. After evaluating sixty-nine reports of Piper's mediumship he considered the hypothesis of telepathy as well as Piper obtaining information about her sitters by natural means such as her memory recalling information. According to James the "spirit-control" hypothesis of her mediumship was incoherent, irrelevant and in cases demonstrably false.

Like Sigmund Freud, James was influenced by Charles Darwin's theory of natural selection. At the core of James' theory of psychology, as defined in The Principles of Psychology (1890), was a system of "instincts". James wrote that humans had many instincts, even more than other animals. These instincts, he said, could be overridden by experience and by each other, as many of the instincts were actually in conflict with each other. In the 1920s, however, psychology turned away from evolutionary theory and embraced radical behaviorism.

In william James's lecture of 1896 titled "The Will to Believe", James defends the right to violate the principle of evidentialism in order to justify hypothesis venturing. This idea foresaw 20th century objections to evidentialism and sought to ground justified belief in an unwavering principle that would prove more beneficial. Through his philosophy of pragmatism william James justifies religious beliefs by using the results of his hypothetical venturing as evidence to support the hypothesis' truth. Therefore, this doctrine allows one to assume belief in a god and prove its existence by what the belief brings to one's life.

William James provided a description of the mystical experience, in his famous collection of lectures published in 1902 as The Varieties of Religious Experience. These criteria are as follows

Following his January, 1907 retirement from Harvard, James continued to write and lecture, publishing Pragmatism, A Pluralistic Universe, and The Meaning of Truth. James was increasingly afflicted with cardiac pain during his last years. It worsened in 1909 while he worked on a philosophy text (unfinished but posthumously published as Some Problems in Philosophy). He sailed to Europe in the spring of 1910 to take experimental treatments which proved unsuccessful, and returned home on August 18. His heart failed on August 26, 1910 at his home in Chocorua, New Hampshire. He was buried in the family plot in Cambridge Cemetery, Cambridge, Massachusetts.

President Jimmy Carter's Moral Equivalent of War Speech, on April 17, 1977, equating the United States' 1970s Energy crisis, oil crisis and the changes and sacrifices Carter's proposed plans would require with the "moral equivalent of war," may have borrowed its title, much of its theme and the memorable phrase from James' classic essay "The Moral Equivalent of War" derived from his last speech, delivered at Stanford University in 1906, and published in 1910, in which "James considered one of the classic problems of politics: how to sustain political unity and civic virtue in the absence of war or a credible threat...." and which "...sounds a rallying cry for Service in the interests of the individual and the nation."

Along with Charles Sanders Peirce and John Dewey, James is considered to be one of the major figures associated with the philosophical school known as pragmatism, and is also cited as one of the founders of functional psychology. A Review of General Psychology analysis, published in 2002, ranked James as the 14th most eminent Psychologist of the 20th century. He also developed the philosophical perspective known as radical empiricism. James' work has influenced intellectuals such as Émile Durkheim, W. E. B. Du Bois, Edmund Husserl, Bertrand Russell, Ludwig Wittgenstein, Hilary Putnam, and Richard Rorty, and has even influenced Presidents, such as Jimmy Carter.

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.

James called compatibilism a "quagmire of evasion", just as the ideas of Thomas Hobbes and David Hume—that free will was simply freedom from external coercion—were called a "wretched subterfuge" by Immanuel Kant.

A Philosopher, according to James, must accept geniuses as a given entity the same way as a Biologist accepts as an entity Darwin's ‘spontaneous variations.’ The role of an individual will depend on the degree of its conformity with the social environment, epoch, moment, etc.