Born in Hampton, New Brunswick, on April 30, 1905, John Humphrey was educated at Mount Allison and McGill University. He married Jeanne Godreau in 1929 and after her death, married Dr. Margaret Kunstler in 1981. Humphrey had one brother, Douglas, and a sister, Ruth. His cousin, Jack Humphrey, became a prominent Canadian painter. After a career in private legal practice in Montreal with Wainwright, Elder and MacDougall, Humphrey joined the McGill teaching staff in 1936 as a sessional lecturer in the Faculty of Law and became the Gale Professor of Law in 1946. Although named as McGill's Dean of Law in 1946 he never served in that function, instead he was drafted by the United Nations to become the first director of the Human Rights Division of the United Nations. On leave from McGill, Humphrey served in his United Nations post until 1966. The author of the first draft of the United Nations Universal Declaration of Human Rights, Humphrey was a key player behind the United Nation's approval of the declaration in 1948. During his tenure as director Humphrey pursued such human rights issues as the situation of women, freedom of the press, and racial discrimination. After his distinguished career with the United Nations, Humphrey returned to teach in McGill's Faculty of Law and continued his advocacy of human rights issues and causes.

Humphrey became an internationally renowned authority on human rights who spoke and wrote frequently on this subject throughout his life. Instrumental in the founding of the Canadian Human Rights Foundation and the Canadian Section of Amnesty International, he also served as president of both organisations. He was a member of the Royal Commission on the Status of Women in Canada and also lobbied extensively for such diverse causes as rights for imprisoned Greek lawyers, Canadian Hong Kong veterans, and Korean "comfort women", or survivors of the Japanese military system of sexual slavery during the Second World War. Humphrey was the author of numerous articles on international politics and legal subjects and the author of two books on the subject of human rights: Human Rights and the United Nations: A Great Adventure, published in 1984, and No Distant Millennium: The International Law of Human Rights, published in 1989.

Humphrey's energetic commitment to various organisations dedicated to human rights and world peace has been acknowledged by numerous honours and awards, including the United Nations Award for outstanding achievement in human rights, Officer of the Order of Canada, and the National Order of Quebec, World Legal Scholar Award, and World Federalists of Canada Peace Prize Award. Recognised as a pioneering figure in the field of human rights in the post-World War II era, John P. Humphrey died on March 14, 1995.