John Mack was born on March 7th, 1942, in New York City to Grace and Lewis Mack. He was the middle of three children and was raised in New Canaan, Connecticut. He attended Phillips Exeter Academy, Princeton University, and then served in Vietnam as an infantry officer in the Marine Corps from 1964-1967.

After returning home, John attended Union Theological Seminary and received his Master of Divinity degree in 1971. While there, he met his future wife, Barbara Gerlach. In 1971, they would serve as co-pastors of Trinity Congregational UCC in Scranton, Pennsylvania. They would move to Boston in 1973 when John was executive and clinical director of Together, Inc., a community outreach program that worked with at-risk teenagers and their families.

In 1984, John started to serve as the pastor of First Congregational Church in Washington, DC, and would spend the next 23 years there. During that time, the church became a just peace, open and affirming, multiracial and multicultural church, and in general a place for all walks of life in downtown Washington. Near the end of his tenure, John helped lead a process for developing the church's site into a mixed-use structure that houses the church and its social service programs, financed by eight floors of offices. John was also a member of the board of directors of the Downtown Cluster of Congregations from 1985-2007, which worked to develop a "lived-in" downtown during the 80s and 90s.

John's family included his wife Barbara, their two children Jessica Prentice and Peter Gerlach-Mack, several grandchildren, his sister Lucy Mack and brother Talbot Mack. While visiting the Himalayas in 2007, John fell ill and spent several weeks in the hospital. He died in his home, surrounded by his family, on January 15th, 2008.