Garten was born to a Jewish family, the son of Ruth (née Engelman) and Melvin Garten. His father fought in World War II, the Korean War, and Vietnam; and was awarded the Distinguished Service Cross in 1953 for his heroism in Korea's Battle of Pork Chop Hill. His brother, Allan Garten, is a retired federal prosecutor in Portland, Oregon. He married Ina Rosenberg in 1968. Garten earned his A.B. from Dartmouth College in 1968 and an M.A. (1972) and Ph.D. (1980) from the Johns Hopkins University's School of Advanced International Studies. He also served in the United States Army from 1968 to 1972, holding the rank of Lieutenant in the 82nd Airborne Division and Captain and aide-de camp to the commanding general of the US Special Forces. In 1971 he was an advisor to the Royal Thai Army.