Born of Irish descent in Oak Park, Illinois, on February 19, 1932, Kerwin graduated from Fenwick High School, a private school in Oak Park, in 1949. He received a Bachelor of Arts degree in Philosophy from the College of the Holy Cross, Worcester, Massachusetts, in 1953; a Doctor of Medicine degree from Northwestern University Medical School, Chicago, Illinois, in 1957; completed his internship at the District of Columbia General Hospital in Washington, D.C.; and attended the United States Navy School of Aviation Medicine at Naval Air Station Pensacola, Florida, being designated a naval flight surgeon in December 1958.