Agnes E. Cantwell was born in New York City and graduated from Bayside High School on Long Island in 1939. She went to night school for one year to earn credit to attend nursing school. In 1944 completed a three-year nursing program at St. John's Hospital in Brooklyn, New York. After her training ended in October 1944, she worked for four months as a registered nurse in St. John’s Hospital.
Cantwell entered the Army Nurse Corps in March 1945. She attended basic training at Fort Dix, New Jersey, and then was very briefly assigned to Mason General Hospital on Long Island. By the end of April 1945 she had been sent to Indiantown Gap, Pennsylvania, to prepare for overseas duty in Europe, but following VE Day the next month, Cantwell was sent to Fort Jackson, South Carolina, to prepare for the Pacific. She departed from Camp Kilmer on Staten Island aboard the hospital ship Stafford and arrived in Manila, Philippines, in early September 1945. She was shifted to different field hospitals throughout the Philippines and then in January 1946 her unit was sent to Japan, where she worked in a station hospital in Yokohama and lived in the Belgian Embassy. Cantwell was discharged at the end of July 1946.
Upon returning to the United States, Cantwell was hospitalized at Tilten General Hospital, Fort Dix, New Jersey, for scabies. After recovering, she enrolled at the Women’s College of the University of North Carolina (now the University of North Carolina at Greensboro), and graduated in 1948 with a bachelor of science in nursing. Cantwell returned to New York City and worked at the Memorial Sloan-Kettering Cancer Center. She met her husband, Richard Cantwell, while he was doing his dental residency at the hospital. They married in 1955 and moved to Washington, D.C. While there, Cantwell worked at the National Cancer Institute in Bethesda, Maryland. After retiring from NCI and having children, she went to work in her husband’s dental office. They retired together in 1992.