The Ebers papyrus (ca 1550 BC) contains a short description of clinical depression. Though full of incantations and foul applications meant to turn away disease-causing demons and other superstition, it also evidences a long tradition of empirical practice and observation.

The root of the word psychology (psyche) means "soul" or "spirit" in Greek, and psychology was sometimes considered a study of the soul (in a religious sense of this term), though its emergence as a medical discipline can be seen in Thomas Willis' reference to psychology (the "Doctrine of the Soul") in terms of brain function, as part of his 1672 anatomical treatise "De Anima Brutorum" ("Two Discourses on the Souls of Brutes").

Until about the end of the 19th century, psychology was regarded as a branch of philosophy.

In 1879 Wilhelm Wundt founded a laboratory at the University in Germany in Leipzig specifically to focus on general and basic questions concerning behaviour and mental states. Then in 1890, William James published the book Principles of Psychology which laid many of the foundations for the sorts of questions that psychologists would focus on for years to come. Crucially, the approach of Wundt and James did not involve metaphysics or religious explanations of human thought and behaviour, freeing it from the realms of philosophy and theology, founding the modern science of psychology.

Meanwhile, Sigmund Freud had developed and applied a method of a method of uncovering repressed wishes known as psychoanalysis. Freud's understanding of the mind was largely based on interpretive methods and introspection (a technique also championed by Wundt), but was particularly focused on resolving mental distress and psychopathology. Freud's theories were wildly successful, not least because they aimed to be of practical benefit to individual patients, but also because they tackled subjects such as sexuality and repression as general aspects of psychological development. These were largely considered taboo subjects at the time, and Freud provided a catalyst for them to be openly discussed in polite society. Although it has become fashionable to discredit many of Freud's more outlandish theories, his application of psychology to clinical work and his more mainstream work have been massively influential.

Partly as a reaction to the subjective and introspective nature of psychology at the time, behaviourism became popular as a guiding psychological theory. Championed by psychologists such as John B. Watson, Edward Thorndike and B. F. Skinner, behaviourism argued that psychology should be a science of behaviour, not the mind, and rejected the idea of internal mental states such as beliefs, desires or goals, believing all behaviour and learning to be a reaction to the environment. In Watson's 1913 paper Psychology As The Behaviourist Views It, he argued that psychology "is a purely objective experimental branch of natural science", "introspection forms no essential part of its methods..." and "The behaviourist... recognizes no dividing line between man and brute".

Behaviourism was the dominant model in psychology for much of the early 20th century, largely due to the creation and successful application (not least of which in advertising) of conditioning theories as scientific models of human behaviour.

However, it became increasingly clear that although behaviourism had made some important discoveries, it was deficient as a guiding theory of human behaviour. Noam Chomsky's review of Skinner's book Verbal Behaviour (that aimed to explain language acquisition in a behaviourist framework) is considered one of the major factors in the ending of behaviourism's reign. Chomsky demonstrated that language could not purely be learnt from conditioning, as people could produce sentences unique in structure and meaning that couldn't possibly be generated solely through experience of natural language, implying that there must be internal states of mind that behaviourism rejected as illusory. Similarly, work by Albert Bandura showed that children could learn by social observation, without any change in overt behaviour, and so must be accounted for by internal representations.

The rise of computer technology also promoted the metaphor of mental function as information processing. This, combined with a scientific approach to studying the mind, as well as a belief in internal mental states, led to the rise of cognitivism as the dominant model of the mind.

Links between brain and nervous system function were also becoming common, partly due to the experimental work of people like Charles Sherrington and Donald Hebb, and partly due to studies of people with brain injury (see cognitive neuropsychology). With the development of technologies for accurately measuring brain function, neuropsychology and cognitive neuroscience have become some of the most active areas in contemporary psychology.

With the increasing involvement of other disciplines (such as philosophy, computer science and neuroscience) in the quest to understand the mind, the umbrella discipline of cognitive science has been created as a means of focusing such efforts in a constructive way.

However, not all psychologists have been happy with what they perceive as mechanical models of the mind and human nature.

Carl Jung, a one-time follower and contemporary of Freud, was instrumental in introducing notions of spirituality into Freudian psychoanalysis (Freud had rejected religion as a mass delusion).

Alfred Adler, after a brief association with Freud's discussion circle, left to forum his own discipline, called Individual (indivisible) Psychology. His influence on contemporary psychology has been considerable, with many approaches borrowing fragments of his theory. A recent rebirth of his legacy, Classical Adlerian Psychology, combines Adler's original theory of personality, style of psychotherapy, and philosophy of living, with Abraham Maslow's vision of optimal functioning.

Humanistic psychology emerged in the 1950s and has continued as a reaction to positivist and scientific approaches to the mind. It stresses a phenomenological view of human experience and seeks to understand human beings and their behaviour by conducting qualitative research. The humanistic approach has its roots in existentialist and phenomenological philosophy and many humanist psychologists completely reject a scientific approach, arguing that trying to turn human experience into measurements strips it of all meaning and relevance to lived existence.

Some of the founding theorists behind this school of thought are Abraham Maslow who formulated a hierarchy of human needs, Carl Rogers who created and developed client centred therapy, and Fritz Perls who helped create and develop Gestalt therapy.