Lemnitzer was born on August 29, 1899 in Honesdale, Pennsylvania. He graduated from West Point in 1920 and was assigned at his request to a Coast Artillery unit. Lemnitzer served in the Philippines but soon began receiving the staff assignments that marked his military career.
Lemnitzer was promoted to Brigadier General in June 1942 and assigned to General Eisenhower's staff shortly thereafter. He helped formed the plans for the invasions of North Africa and Sicily and was promoted to Major General in November 1944. Lemnitzer was one of the senior officers sent to negotiate the Italian surrender in 1943 and the German surrender in 1945. He would later be accused of making it possible for some Nazis to elude investigations for war crimes.
Following the end of World War II, Lemnitzer was assigned to the Strategic Survey Committee of the Joint Chiefs of Staff and was later named as Deputy Commandant of the National War College. In 1950, at the age of 51, he took parachute training and was subsequently placed in command of the 11th Airborne Division. He was assigned to Korea in command of the 7th Infantry Division in November 1951 and was promoted to Lieutenant General in August 1952.
Lemnitzer was promoted to the rank of General and named Commander of U.S. Army Forces in the Far East and of the 8th Army in March 1955. He was named Chief of Staff of the Army in July 1957 and he was appointed as Chairman of the Joint Chiefs of Staff in September 1960. As Chairman, Lemnitzer weathered the Bay of Pigs crisis and the early years of American involvement in Vietnam. He was also required to testify before the Senate Foreign Affairs Committee about his knowledge of the activities of Major General Edwin Walker, an extreme right wing racist who had been dismissed from the Army over alleged attempts to promote his beliefs in the military. Lemnitzer led the creation of the controversial plans for Operation Northwoods in 1962; a proposed (and rejected) plan to discredit the Castro regime and create support for military action against Cuba by staging anti-American incidents.
Lemnitzer was named as Commander of U.S. Forces in Europe in November 1962 and was appointed as Supreme Allied Commander of NATO in January 1963. Once again, he was called upon to lead in difficult times as this period encompassed the Cyprus crisis of 1963-1964 and the withdrawal of NATO forces from France in 1966.
Lemnitzer retired from the military in July 1969. In 1975, President Ford appointed Lemnitzer to the Commission on CIA Activities within the United States (aka the Rockefeller Commission) to investigate whether the Central Intelligence Agency had committed acts that violated American laws.
Lemnitzer died on November 12, 1988 and is buried in Arlington National Cemetery. His wife, Katherine Tryon Lemnitzer (1901-1994), is buried with him.