[4] In 1969, Sommer joined the Public Health Service as an Epidemic Intelligence Service (EIS) officer at the Centers for Disease Control and Prevention, and moved overseas with his family to work in the Cholera Research Laboratory in Dhaka, Bangladesh (then known as East Pakistan),[6] where he conducted the first formal epidemiologic investigation of a major disaster: the 1970 cyclone that washed away a quarter of a million people in a single night.

Upon completing his Master of Health Sciences degree in epidemiology there, Sommer spent three years as a resident and fellow in ophthalmology at the Wilmer Eye Institute (associated with the Johns Hopkins School of Medicine) from 1973 to 1976.

Sommer is currently a Johns Hopkins University Distinguished Service Professor,[13] inaugural Gilman Scholar,[14] and Dean Emeritus of the Bloomberg School of Public Health.

[12] In the mid-1980s, Sommer initiated and led the development of one of the first, and still rigorously updated, clinical guidelines of any medical specialty: the "Preferred Practice Patterns" of the American Academy of Ophthalmology.

Sommer conducted a sequence of observational and intervention trials in Indonesia, and subsequently elsewhere, that led to his discovery that Vitamin A deficiency reduces immune responsiveness, and therefore resistance to deadly infectious diseases, especially diarrhea and measles.

[1][2][20] Sommer made a number of other discoveries that have led to major advances in global health care and policies, including demonstrating that measurement of mid-arm-circumference (MUAC) is a simple and effective tool for conducting nutritional surveillance and identifying children and populations at high risk of dying from malnutrition;[21] that the easily assessed appearance of the nerve fiber layer in the retina is an early, accurate predictor of glaucomatous optic nerve damage indicating the need to initiate glaucoma therapy;[22] and that vaccination for smallpox as long as 6 days after infection can prevent the disease,[23] an observation that forestalled mass vaccination of primary responders following 9/11.

Sommer's current research interests include the diagnosis and management of glaucoma, improved child survival and blindness prevention strategies, and micronutrient interventions, in addition to other projects in both epidemiology and ophthalmology.