Endicott Peabody, nicknamed "Chub", was born in Lawrence, Massachusetts, the son of Mary Elizabeth (née Parkman) and the Rev. Malcolm E. Peabody, a former Episcopal Bishop of Central New York. He was a grandson of the founder of Groton School and Brooks School, also named Endicott Peabody. He earned his A.B. from Harvard College in 1942. An All-America star defensive lineman for the team, he was later inducted into the .
Peabody served in the United States Navy during World War II. He received the Silver Star for gallantry for service as a Lieutenant aboard the USS Tirante in the Pacific Ocean theater.
After the war, Peabody attended Harvard Law School, receiving his J.D. degree and attaining admission to the Massachusetts bar in 1948. He was Assistant Regional Counsel for the Office of Price Stabilization and Regional Counsel for the Small Defense Plants Administration in the early 1950s. From 1954 to 1956 he served on the Massachusetts Governor's Council. During the 1960 presidential election, he coordinated John F. Kennedy's campaigns in West Virginia, Pennsylvania, and New Hampshire.
In 1958, Peabody ran for Attorney General of Massachusetts, but lost in the Democratic primary to Edward McCormack, Jr. by nine percentage points.
In 1960, Peabody ran for Governor of Massachusetts, but came in second (out of seven candidates) in the Democratic primary with 25.5% of the vote.