Hassler was born in Allentown, Pennsylvania in the United States. He grew up in New York and was educated at Brooklyn Polytechnic Institute. He studied night classes in journalism at Columbia University. He became employed by the Leader-Observer in Queens and then American Baptist Publications in Philadelphia.
In 1942, Alfred Hassler became the editor of a journal called Fellowship. Following this he was imprisoned for his stance as a conscientious objector during World War II. While imprisoned, he wrote a book, Diary of a Self-Made Convict. He authored several anti-war books and articles. Hassler was an active member of the peace group Fellowship of Reconciliation and a friend of Thich Nhat Hanh. Hassler led a FoR delegation to Vietnam in 1969. Hassler supported the Vietnamese Buddhists, arguing they could form a nonviolent "Third Force" for peace independent of both the Saigon and Hanoi governments.  Hassler retired from his position with the fellowship and, in 1974, with his wife Dorothy founded a retirement community in southern Spain, Almeria. In the 1980s he returned to New York.
Hassler also founded the Dai Dong project which linked war, environmental issues and poverty, and he was the president of the International Confederation for Disarmament and Peace.
- Peoples congress - Registry of World Citizens
- Mary Hershberger, Traveling to Vietnam: American Peace Activists and the War. Syracuse University Press, 1998 ISBN 081560517X, (p.21, 157)
- Lifelong Pacifist And Environmentalist, Dies at 81 - New York Times
- Sperry, Rod Meade (May 2013), "3 Heroes, 5 Powers", Shambhala Sun 21 (5): 68–73