Bowlby was born in London to an upper-middle-class family. He was the fourth of six children and was brought up by a nanny in the British fashion of his class at that time. His father, Sir Anthony Bowlby, first Baronet, was surgeon to the King's Household, with a tragic history: at age five, Sir Anthony's own father (John's grandfather) was killed while serving as a war correspondent in the Opium Wars.

Normally, Bowlby saw his mother only one hour a day after teatime, though during the summer she was more available. Like many other mothers of her social class, she considered that parental attention and affection would lead to dangerous spoiling of the children. Bowlby was lucky in that the nanny in his family was present throughout his childhood. When Bowlby was almost four years old, his beloved nanny, who was actually his primary caretaker in his early years, left the family. Later, he was to describe this as tragic as the loss of a mother.

At the age of seven, he was sent off to boarding school, as was common for boys of his social status. In his work Separation: Anxiety and Anger, he revealed that he regarded it as a terrible time for him. He later said, "I wouldn't send a dog away to boarding school at age seven". Because of such experiences as a child, he displayed a sensitivity to children’s suffering throughout his life. However, with his characteristic attentiveness to the effects of age differences, Bowlby did consider boarding schools appropriate for children aged eight and older, and wrote, "If the child is maladjusted, it may be useful for him to be away for part of the year from the tensions which produced his difficulties, and if the home is bad in other ways the same is true. The boarding school has the advantage of preserving the child's all-important home ties, even if in slightly attenuated form, and, since it forms part of the ordinary social pattern of most Western communities today [1951], the child who goes to boarding-school will not feel different from other children. Moreover, by relieving the parents of the children for part of the year, it will be possible for some of them to develop more favorable attitudes toward their children during the remainder."

He married Ursula Longstaff, herself the daughter of a surgeon, on April 16, 1938, and they had four children, including (Sir) Richard Bowlby, who succeeded his uncle as third Baronet.

Bowlby died at his summer home on the Isle of Skye, Scotland.

Bowlby studied psychology and pre-clinical sciences at Trinity College, Cambridge, winning prizes for outstanding intellectual performance. After Cambridge, he worked with maladjusted and delinquent children, then at the age of twenty-two enrolled at University College Hospital in London. At the age of twenty-six, he qualified in medicine. While still in medical school he enrolled himself in the Institute for Psychoanalysis. Following medical school, he trained in adult psychiatry at the Maudsley Hospital. In 1937, aged 30, he qualified as a psychoanalyst.

During World War II, he was a Lieutenant Colonel in the Royal Army Medical Corps. After the war, he was Deputy Director of the Tavistock Clinic, and from 1950, Mental Health Consultant to the World Health Organization.

Because of his previous work with maladapted and delinquent children, he became interested in the development of children and began work at the Child Guidance Clinic in London. This interest was probably increased by a variety of wartime events involving separation of young children from familiar people; these included the rescue of Jewish children by the Kindertransport arrangements, the evacuation of children from London to keep them safe from air raids, and the use of group nurseries to allow mothers of young children to contribute to the war effort. Bowlby was interested from the beginning of his career in the problem of separation and the wartime work of Anna Freud and Dorothy Burlingham on evacuees and Rene Spitz on orphans. By the late 1950s he had accumulated a body of observational and theoretical work to indicate the fundamental importance for human development of attachment from birth.

Bowlby was interested in finding out the actual patterns of family interaction involved in both healthy and pathological development. He focused on how attachment difficulties were transmitted from one generation to the next. In his development of attachment theory he propounded the idea that attachment behaviour was essentially an evolutionary survival strategy for protecting the infant from predators. Mary Ainsworth, a student of Bowlby’s, further extended and tested his ideas, and in fact played the primary role in suggesting that several attachment styles existed.The three most important experiences for Bowlby’s future work and the development of attachment theory were his work with:

• Maladapted and delinquent children.

• James Robertson (in 1952) in making the documentary film A Two-Year Old Goes to the Hospital, which was one of the films about ”young children in brief separation“. The documentary illustrated the impact of loss and suffering experienced by young children separated from their primary caretakers. This film was instrumental in a campaign to alter hospital restrictions on visiting by parents. In 1952 when he and Robertson presented their film A Two Year Old Goes to Hospital to the British Psychoanalytical Society, psychoanalysts did not accept that a child would mourn or experience grief on separation but instead saw the child's distress as caused by elements of unconscious fantasies (in the film because the mother was pregnant).

• Melanie Klein during his psychoanalytic training. She was his supervisor; however they had different views about the role of the mother in the treatment of a three-year-old boy. Specifically and importantly, Klein stressed the role of the child's fantasies about his mother, but Bowlby emphasized the actual history of the relationship. Bowlby's views...that children were responding to real life events and not unconscious fantasies...were rejected by psychoanalysts, and Bowlby was effectively ostracized by the psychoanalytic community. He later expressed the view that his interest in real-life experiences and situations was "alien to the Kleinian outlook".

In 1949, Bowlby's earlier work on delinquent and affectionless children and the effects of hospitalised and institutionalised care lead to his being commissioned to write the World Health Organization's report on the mental health of homeless children in post-war Europe. The result was Maternal Care and Mental Health published in 1951.

Bowlby drew together such limited empirical evidence as existed at the time from across Europe and the USA. His main conclusions, that “the infant and young child should experience a warm, intimate, and continuous relationship with his mother (or permanent mother substitute) in which both find satisfaction and enjoyment” and that not to do so may have significant and irreversible mental health consequences, were both controversial and influential. The 1951 WHO publication was highly influential in causing widespread changes in the practices and prevalence of institutional care for infants and children, and in changing practices relating to the visiting of infants and small children in hospitals by parents. The theoretical basis was controversial in many ways. He broke with psychoanalytic theories which saw infants' internal life as being determined by fantasy rather than real life events. Some critics profoundly disagreed with the necessity for maternal (or equivalent) love in order to function normally, or that the formation of an ongoing relationship with a child was an important part of parenting. Others questioned the extent to which his hypothesis was supported by the evidence. There was criticism of the confusion of the effects of privation (no primary attachment figure) and deprivation (loss of the primary attachment figure) and in particular, a failure to distinguish between the effects of the lack of a primary attachment figure and the other forms of deprivation and understimulation that may affect children in institutions.

The monograph was also used for political purposes to claim any separation from the mother was deleterious in order to discourage women from working and leaving their children in daycare by governments concerned about maximising employment for returned and returning servicemen. In 1962 WHO published Deprivation of maternal care: A Reassessment of its Effects to which Mary Ainsworth, Bowlby's close colleague, contributed with his approval, to present the recent research and developments and to address misapprehensions. This publication also attempted to address the previous lack of evidence on the effects of paternal deprivation.

According to Rutter the importance of Bowlby's initial writings on 'maternal deprivation' lay in his emphasis that children's experiences of interpersonal relationships were crucial to their psychological development.

Bowlby himself explained in his 1988 work "A Secure Base" that the data were not, at the time of the publication of Maternal Care and Mental Health, "accommodated by any theory then current and in the brief time of my employment by the World Health Organization there was no possibility of developing a new one". He then went on to describe the subsequent development of attachment theory. Because he was dissatisfied with traditional theories, Bowlby sought new understanding from such fields as evolutionary biology, ethology, developmental psychology, cognitive science and control systems theory and drew upon them to formulate the innovative proposition that the mechanisms underlying an infants tie emerged as a result of evolutionary pressure. "Bowlby realised that he had to develop a new theory of motivation and behaviour control, built on up-to-date science rather than the outdated psychic energy model espoused by Freud." Bowlby expressed himself as having made good the "deficiencies of the data and the lack of theory to link alleged cause and effect" in Maternal Care and Mental Health in his later work Attachment and Loss published in 1969.

"From the 1950s Bowlby was in personal and scientific contact with leading European scientists in the field of ethology, namely Niko Tinbergen, Konrad Lorenz, and especially the rising star of ethology Robert Hinde. Using the viewpoints of this emerging science and reading extensively in the ethology literature, Bowlby developed new explanatory hypotheses for what is now known as human attachment behaviour. In particular, on the basis of ethological evidence he was able to reject the dominant Cupboard Love theory of attachment prevailing in psychoanalysis and learning theory of the 1940s and 1950s. He also introduced the concepts of environmentally stable or labile human behaviour allowing for the revolutionary combination of the idea of a species-specific genetic bias to become attached and the concept of individual differences in attachment security as environmentally labile strategies for adaptation to a specific childrearing niche. Alternately, Bowlby’s thinking about the nature and function of the caregiver-child relationship influenced ethological research, and inspired students of animal behaviour such as Tinbergen, Hinde, and Harry Harlow. Bowlby spurred Hinde to start his ground breaking work on attachment and separation in primates (monkeys and humans), and in general emphasized the importance of evolutionary thinking about human development that foreshadowed the new interdisciplinary approach of evolutionary psychology. Obviously, the encounter of ethology and attachment theory led to a genuine cross-fertilization" (Van der Horst, Van der Veer & Van IJzendoorn, 2007, p. 321).

Before the publication of the trilogy in 1969, 1972 and 1980, the main tenets of attachment theory, building on concepts from ethology and developmental psychology, were presented to the British Psychoanalytical Society in London in three now classic papers: The Nature of the Child’s Tie to His Mother (1958), Separation Anxiety (1959), and Grief and Mourning in Infancy and Early Childhood (1960). Bowlby rejected psychoanalyst explanations for attachment, and in return, psychoanalysts rejected his theory. At about the same time, Bowlby's former colleague, Mary Ainsworth was completing extensive observational studies on the nature of infant attachments in Uganda with Bowlby's ethological theories in mind. Her results in this and other studies contributed greatly to the subsequent evidence base of attachment theory as presented in 1969 in Attachment the first volume of the Attachment and Loss trilogy. The second and third volumes, Separation: Anxiety and Anger and Loss: Sadness and Depression followed in 1972 and 1980 respectively. Attachment was revised in 1982 to incorporate recent research.

According to attachment theory, attachment in infants is primarily a process of proximity seeking to an identified attachment figure in situations of perceived distress or alarm for the purpose of survival. Infants become attached to adults who are sensitive and responsive in social interactions with the infant, and who remain as consistent caregivers for some months during the period from about 6 months to two years of age. Parental responses lead to the development of patterns of attachment which in turn lead to 'internal working models' which will guide the individual's feelings, thoughts, and expectations in later relationships. In Bowlby's approach, the human infant is considered to have a need for a secure relationship with adult caregivers, without which normal social and emotional development will not occur.

As the toddler grows, it uses its attachment figure or figures as a "secure base" from which to explore. Mary Ainsworth used this feature plus "stranger wariness" and reunion behaviours, other features of attachment behaviour, to develop a research tool called the "Strange Situation Procedure" for developing and classifying different attachment styles.

The attachment process is not gender specific as infants will form attachments to any consistent caregiver who is sensitive and responsive in social interactions with the infant. The quality of the social engagement appears to be more influential than amount of time spent.

Bowlby's last work, published posthumously, is a biography of Charles Darwin,which discusses Darwin's "mysterious illness" and whether it is psychosomatic.

Although not without its critics, attachment theory has been described as the dominant approach to understanding early social development and to have given rise to a great surge of empirical research into the formation of children's close relationships. As it is presently formulated and used for research purposes, Bowlby's attachment theory stresses the following important tenets:

1) Children between 6 and about 30 months are very likely to form emotional attachments to familiar caregivers, especially if the adults are sensitive and responsive to child communications.

2) The emotional attachments of young children are shown behaviourally in their preferences for particular familiar people, their tendency to seek proximity to those people, especially in times of distress, and their ability to use the familiar adults as a secure base from which to explore the environment.

3) The formation of emotional attachments contributes to the foundation of later emotional and personality development, and the type of behaviour toward familiar adults shown by toddlers has some continuity with the social behaviours they will show later in life.

4) Events that interfere with attachment, such as abrupt separation of the toddler from familiar people or the significant inability of carers to be sensitive, responsive or consistent in their interactions, have short-term and possible long-term negative impacts on the child's emotional and cognitive life.