Daly's father had immigrated from the British West Indies, found work as a postal clerk, and eventually married Helen Page of Washington, D.C. They lived in New York, and Daly was born and raised in Corona, Queens. She often visited her maternal grandparents in Washington, where she was able to read about scientists and their achievements in her grandfather’s extensive library. She was especially impressed by Paul de Kruif’s The Microbe Hunters, a work which partially influenced her decision to become a scientist.
Daly’s father, who had attended Cornell University with intentions of becoming a chemist, was unable to complete his education due to a lack of funds. His daughter continued her father’s legacy by majoring in chemistry.
After Daly graduated from all-girls Hunter College High School (where she was also encouraged to pursue chemistry), she enrolled in Queens College, a small, fairly new school in Flushing, New York. She lived at a home to save money, majored in chemistry, and graduated from Queens magna cum laude with her bachelor's degree in chemistry in 1942.
Daly remained at Queens College for another year, working as a laboratory assistant while attending graduate school at New York University. She completed her masters in Chemistry in 1 year. She then enrolled in the doctoral program at Columbia University, where she was supervised by Dr. Mary L. Caldwell. Caldwell, who had a doctorate in nutrition, helped Daly discover how chemicals produced in the body contribute to food digestion. Daly completed her Ph.D. in chemistry in 1947. Her dissertation addressed “A Study of the Products Formed by the Action of Pancreatic Amylase on Corn Starch.”
Daly served two years as a physical science instructor at Howard University. After being awarded an American Cancer Society grant to support her postdoctoral research, she joined Dr. A. E. Mirsky at the Rockefeller Institute, where they studied the cell nucleus. In 1953, after Watson and Crick described the structure of DNA, Daly’s world changed significantly: suddenly, the cell nucleus research field was flooded with funding opportunities. Her work flourished in the new environment.
In 1955 Daly returned to Columbia’s College of Physicians and Surgeons to teach biochemistry. She began collaborating with Dr. Quentin B. Deming to investigate the underlying causes of heart attacks. They found that high cholesterol levels contributed to the blockage of arteries that supply oxygen to the heart. She also investigated the effects of sugar on the function of coronary arteries. Later, she became a pioneer in studying the impact of cigarette smoking on the lungs.
In 1960 Daly and Deming moved to Yeshiva University at the Albert Einstein College of Medicine in New York City. At Yeshiva, she continued her research and taught biochemistry courses. She enjoyed teaching medical students and was dedicated to increasing the number of minority students enrolled in medical schools. In 1961 Daly married Vincent Clark.
Daly also served as an investigator for the American Heart Association; she was especially interested in how hypertension affects the circulatory system. She was a member of the prestigious Board of Governors of the New York Academy of Sciences for two years. Daly retired from the Einstein College of Medicine in 1986, and in 1988 she established a scholarship for African American chemistry and physics majors at Queens College in memory of her father.