Upon his return to Boston, Dr. Cannon became an assistant surgeon at the Massachusetts General Hospital. He joined Dr. Kazanjian in the plastic clinic, where he applied for the assignment on burns. In 1941, Drs. Cannon and Kazanjian, at the request of the Massachusetts Department of Public Health, started an autonomous clinic at the Mount Auburn Hospital in Cambridge under the Crippled Children's Program. This program, which treated children with congenital conditions such as cleft lips, lasted until 1974.

In 1943, Dr. Cannon assumed active duty in the Medical Corps of the United States Army and established the plastic unit at the Valley Forge General Hospital. In 1947, Dr. Cannon returned to Boston and resumed his practice, which he continued until his retirement in 1989. During the post-World War II years, Dr. Cannon was a major influence in the establishment of plastic surgery as a recognized specialty in Boston. The clinic at the Mount Auburn Hospital was re-established. Dr. Cannon was called on as a surgical consultant throughout Eastern Massachusetts, showing some of the finer points of technique in treating wounds. He also served as an advisor in the fracture clinic at the Massachusetts General Hospital, on the management of associated soft tissue damage.

Dr. Cannon served on the faculty at the Harvard Medical School, where he retired as Clinical Professor of Surgery emeritus in 1974. In 1991, he was named Honorary Surgeon at the Massachusetts General Hospital. Dr. Cannon also served as president of the New England Society of Plastic Surgeons and the American Association of Plastic Surgeons. As a fellow of the American College of Surgeons and a leader in the professional organizations of plastic surgeons, Dr. Cannon was able to bring legitimacy into the specialty. Dr. Cannon's hard work was finally realized when the Massachusetts General Hospital established its first residency for plastic surgery in 1970.