Leonard Feinberg was born on September 7, 1923 in Long Branch, New Jersey. He is descended from Polish and Russian Jews who emigrated in the 1890s. When he was very young, his parents moved to Elizabeth, New Jersey, where he grew up and went to Abraham Lincoln School #14. After graduating from Jefferson High School in 1941, Feinberg commuted to Rutgers College and majored in biology.
With World War II raging, Feinberg enlisted in the Army in 1942. He went to Surgical Technicians School at Camp Grant, before being shipped overseas to Europe. Feinberg served in the medical detachment of the First Battalion, 134th Infantry Regiment of the Army's 35th Infantry Division. In the immediate aftermath of the Allied invasion of German-occupied France on June 6, 1944, Feinberg's unit landed in Normandy in July 1944 and partook in the offensive against German forces in France at Saint-Lô, in the Mortain-Falaise Gap, and at Nancy, where he was wounded after being hit by pieces of shrapnel. After rejoining his unit, Feinberg saw action in Germany, Luxembourg and Belgium. During the Battle of the Bulge, Feinberg was captured by German forces and was a prisoner-of-war until April 1944.
After the war, Feinberg used the GI Bill and returned to Rutgers, graduating in 1947. He married Elizabeth native Abbey Levine Feinberg, and they had two children, Melinda and Brian. Feinberg spent his career in the pharmaceutical industry, becoming the director and owner of Clinical Laboratories.