The son of a middle-class lawyer—who was also a journalist and local political figure—and of a mother belonging to the same social background, Montini was in his early years educated mainly at home because of frail health. Later he studied in Brescia. Ordained priest on May 29, 1920, he was sent by his bishop to Rome for higher studies and was eventually recruited for the Vatican diplomatic service. His first assignment, in May 1923, was to the staff of the apostolic nunciature (papal ambassador's post) in Warsaw, but persistent ill health brought him back to Rome before the end of that same year. He then pursued special studies at the Ecclesiastical Academy, the training school for future Vatican diplomats, and at the same time resumed work at the Vatican Secretariat of State, where he remained in posts of increasing importance for more than 30 years.

In 1939 Montini was appointed papal undersecretary of state and later, in 1944, acting secretary for ordinary (or nondiplomatic) affairs. He declined an invitation to be elevated to the Sacred College of Cardinals in 1953. In the beginning of November 1954, Pope Pius XII appointed him archbishop of Milan, and Pope John XXIII named him cardinal in 1958. He was elected pope on June 21, 1963, choosing to be known as Paul VI.

The Montini pontificate began in the period following the difficult first session of the second Vatican Council, in which the new pope had played an important, though not spectacular, part. His lengthy association with university students in the stormy atmosphere of the early days of the Fascist regime in Italy, in combination with the generally philosophical bent of his mind—developed by a long-standing habit of extensive and reflective reading—enabled him to bring to the perplexing problems of the times an academic understanding, coupled with the knowledge derived from long years of practical diplomatic experience. Paul VI guided the three remaining sessions of the second Vatican Council, often developing points he had first espoused as cardinal archbishop of Milan. His chief concern was that the Roman Catholic church in the 20th century should be a faithful witness to the tradition of the past, except when tradition was obviously anachronistic.

Upon the completion of the council (December 8, 1965), Paul VI was confronted with the formidable task of implementing its decisions, which affected practically every facet of church life. He approached this task with a sense of the difficulty involved in making changes in centuries-old structures and practices—changes rendered necessary by many rapid transformations in the social, psychological, and political milieu of the 20th century. Paul VI's approach was consistently one of careful assessment of each concrete situation, with a sharp awareness of the many varied complications that he believed could not be ignored.

This prevalently philosophical attitude was often construed by his critics as timidity, indecision, and uncertainty. Nonetheless, many of Paul VI's decisions in these crucial years called for courage. In July 1968, he published his encyclical Humanae vitae (“Of Human Life”), which reaffirmed the stand of several of his predecessors on the long-smoldering controversy over artificial means of birth prevention (birth control), which he opposed. In many sectors this encyclical provoked adverse reactions that may be described as the most violent attacks on the authority of papal teaching (papal infallibility) in modern times. Similarly, his firm stand on the retention of priestly (priest) celibacy (Sacerdotalis caelibatus, June 1967) evoked much harsh criticism. Paul VI later likened the large numbers of priests leaving the ministry to a “crown of thorns.” He also was disturbed by the growing numbers of religious men and women asking for release from vows or who were abandoning out of hand their religious vows.

From the very outset of his years as pope, Paul VI gave clear evidence of the importance he attached to the study and the solution of social problems and to their impact on world peace. Social questions had already been prominent in his far-reaching pastoral program in Milan (1954–63). During those years he had traveled extensively in the Americas and in Africa, centring his attention mainly on concern for workers and for the poor. Such problems dominated his first encyclical letter, Ecclesiam suam (“His Church”), August 6, 1964, and later became the insistent theme of his celebrated Populorum progressio (“Progress of the Peoples”), March 26, 1967. This encyclical was such a pointed plea for social justice that in some conservative circles the pope was accused of Marxism.

In an address to the council fathers at the end of the first session of the second Vatican Council, the then Cardinal Montini formulated a question that may be called the theme of his pastoral service as pontiff: “Church of Christ, what say you of yourself?” In an effort to answer this fundamental question, Paul VI undertook a series of apostolic journeys that were unparalleled occasions for a pope to set foot on every continent. His first journey was a pilgrimage to the Holy Land (January 1964), highlighted by his historic meeting with the Greek Orthodox patriarch of Constantinople, Athenagoras (Athenagoras I), in Jerusalem. At the end of that same year, he went to India, the first pope to visit Asia. The following year (October 4, 1965) he traveled to the headquarters of the United Nations in New York City, where he delivered a moving plea for peace to the General Assembly in special session. In 1967 he undertook short visits to Fátima (Portugal) and to Istanbul and Ephesus (Turkey), a journey that had special ecumenical (ecumenism) significance: a second meeting with Athenagoras in the patriarch's own episcopal city (Constantinople). In August 1968 the pope went to Bogotá, and he appeared before the International Labour Organisation and the World Council of Churches in Geneva, in June 1969. The following month he was in Uganda, East Africa. In the autumn of 1970, he undertook the longest papal journey in modern history: 10 days spent in visits to Tehrān, East Pakistan, the Philippines, Western Samoa (now Samoa), Australia, Indonesia, Hong Kong, and Ceylon, each stop bringing Paul VI into personal contact with different peoples of the world. His arrival in Manila almost ended in tragedy when an attempt was made on his life within minutes of his descent from the plane, but with no serious injury.

The themes treated by Paul VI on these trips were basically the same: world peace, social justice, world hunger, illiteracy, the brotherhood of man under God, and international cooperation.

On January 6, 1971, in the Clementine Hall in the Vatican, Paul VI conferred the Pope John XXIII Peace Prize on the Albanian-born Mother Mary Teresa Bojaxhiu (Teresa, Blessed Mother), who had spent most of her life in India, where she had founded a special religious congregation of women dedicated to the alleviation of the countless ills of the poorest classes in the country. Paul VI declared on this occasion that the award was intended to centre attention on how even a humble individual without means can further world peace without fanfare, simply by proving in day-to-day action that “every man is my brother.” Here, as in other instances, Paul's aim was to confront the world at large with the inescapable problems of justice and peace while at the same time proving conclusively that even these apparently insoluble problems can and must be settled with realistic courage and individual perseverance.

Paul VI's human concern found further expression in his efforts to lessen the long-standing tensions between the church of Rome and other churches and even with those professing no religion at all. He sought out closer understanding with numerous religious leaders throughout the world, both Christian and non-Christian, placing more emphasis on those aspects that unite the churches than on those that divide. To show that mutual acquaintance is at the very foundation of any plans or hopes for unity, Pope Paul met with prominent religious leaders from various communities in Great Britain, the United States, and the Soviet Union, as well as other countries. Paul VI also set up a special secretariat for nonbelievers, stressing the need of understanding and endeavouring to solve the problems posed by atheism.

Under his guidance the Roman Catholic church drastically revised its legislation governing marriages (marriage) between its own members and those who profess other faiths, expressing a firm desire to diminish the threat of human tragedy following possible clashes of individual consciences. For this reason Paul VI's motu proprio (a type of papal document) was welcomed and praised for its understanding of human problems and its desire to find a satisfactory solution to the problem of mixed marriages without demanding of either side any renunciation of basic principles of conscience.

In the rise of modern ecumenism, Paul VI saw excellent opportunities to encourage world brotherhood, which, he hoped, might enable all men to continue their efforts for human well-being in their pursuit of happiness in unity of faith in God. On May 15, 1971, commemorating the 80th anniversary of Pope Leo XIII's encyclical Rerum novarum on the reform of the social order, Pope Paul issued a forceful apostolic letter, Octogesima adveniens with particular insistence on the necessity of involvement of all men in the solution of the problems of justice and peace.