Dr. Hasselgren has practiced surgery for 45 years. Since 2002, his surgical practice has been at the Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, where he was the Director of Endocrine Surgery and Vice Chairman for Research 2002-2018. He is the George H.A. Clowes, Jr. Distinguished Professor of Surgery at the Harvard Medical School.

Dr. Hasselgren has a long-standing research interest and has conducted NIH-supported research for almost 30 years. His research has resulted in multiple scientific publications related to both basic and clinical research.

Dr. Hasselgren recently published a book, Revolutionary Surgeons. Patriots and Loyalists on the Cutting Edge. The book narrates the lives of surgeons involved in the American Revolutionary War both politically and militarily, while at the same time being successful surgeons.