Choosing a military career, he spent a year of study beyond high school to prepare for a U. S. Military Academy appointment, which he received in June 1918. He graduated in 1920, 86th in a class of 271, with physics his best subject. His education at the Army Command and General Staff College turned him into a planner, he later maintained, and was followed by a tour as an instructor in tactics at the CA School. In 1940 he graduated from the Army War College.
Vicissitude was a hallmark of Lemnitzer's early career, but he would later say that nothing in the service ever caused him to "blow his top" or even consider it. Yet after the class of 1920 were "graduated cadets" and appointed first lieutenants, he and many of his classmates were, eight months later, reduced to second lieutenants. Commissioned in the Coast Artillery Corps (CAC), his early career saw him posted to the Coast Artillery (CA) School and to operational assignments in the Philippines and in Naragansett Bay, 1921-1934. He did two faculty tours at West Point (1926-1930) and (1934 - 1935) teaching physics, mechanics, and hydraulics. During his first CA School tour, Lemnitzer spent a great deal of time studying military history and theory. On his first tour in the Philippines he met Lt. Col. Stanley D. Embick (later brigadier general) who had been at Versailles in 1919 and from whom he learned much about the conduct of diplomacy.
As World War II approached, Lemnitzer was prepared to work as a planner and staff officer, having been promoted to major in July 1940. After a series of command and staff assignments (1940 - 1941), he went to the War Plans Division, then Army General Headquarters and Headquarters Army Ground Forces appointments (May 1941 - July 1942), by the end of which he was a brigadier general. Having taken command of the 34th Antiaircraft Brigade, his arrival in the United Kingdom saw his planning background and his affiliations catch up to him when Eisenhower made him assistant chief of staff for plans and operations at Allied Forces Headquarters. He helped plan "TORCH" and went on Clark's dangerous negotiations mission to French North Africa (October 1942), which marked the beginning of his soldier-diplomat reputation. For his assistance to French resistance leaders who were fighting the Axis-controlled Vichy government, Lemnitzer was given the the Legion of Merit by the French government after the war. There followed command and staff assignments in Tunisia, Sicily, and Italy, finishing as deputy chief of staff to Supreme Allied Command (SAC) Mediterranean, then headed by Gen. Sir Harold Alexander. In that capacity he went to Switzerland to negotiate the German surrender in Italy with Nazi SS General Karl Wolff in March-April 1945. He served as chief of staff to SAC Mediterranean before being appointed Army member of the Joint Strategic Survey Committee. He pressed for jointness in the services postwar and came to believe that most future wars would be coalition wars. He had gained the great respect of U. S. allies during the war and probably modelled himself on Eisenhower and Alexander, the two generals whom he most admired. Lemnitzer became the first deputy commandant of the National War College (1947) and was selected to head the U. S. military delegation to set up the North Atlantic Treaty Organization (NATO) (Summer 1948). He then headed the Office of Military Assistance, coordinating aid to U. S. allies, and he frequently appeared before Congress (1948 - 1950). Believing ever more strongly in coalition warfare, he pushed for policies that diverged from traditional U. S. isolationist views. His advocates, seeing a future for him, saw that he went to jump school (aged 51); commanded the 11th Airborne Division, 7th Infantry Division in Korea (where he received the Silver Star for gallantry); and on to Army deputy chief for plans and research. He returned to the Far East as commander of the 8th Army and Army Ground Forces (March 1955), and then he succeeded Gen. Maxwell Taylor as commander-in-chief, Far East, United Nations Command and governor-general of the Ryukyus. Lemnitzer's diplomatic skills aided him greatly in dealing with Syngman Rhee of Korea, the Ryukyus legislature, and the Japanese government during these years. These skills were most severely tried during the implementation of the Status of Forces Agreement allowing U. S. servicemen accused of civil law violations to be tried in Japanese courts. Lemnitzer displayed remarkable force of character and integrity during the crisis occasioned by the first trial of a serviceman for murder in Japanese courts. He came home to be vice-chief of staff (1957 - 1959). Shortly before leaving office, President Eisenhower selected Lemnitzer to be chairman of the Joint Chiefs of Staff. Lemnitzer at once undertook the coordination of American nuclear forces in the face of a growing Soviet threat.
When President Kennedy altered the plans for the Bay of Pigs operation without reference to the Joint Chiefs of Staff, Lemnitzer was out of the country on an around-the-world trip. On his return, he "ordered" the chiefs to say nothing, in spite of galling criticism, because U. S. allies were concerned more about governmental division than the failure of the effort. In 1962 the Europeans, through Ambassador James Gavin, asked for Lemnitzer as NATO's supreme commander. He was also President Kennedy's personal choice, although his appointment was delayed by the Cuban Missile Crisis. He served from 1963 until 1969, the longest tour of any chief commander in that post. As he did throughout his career, he pursued a pragmatic course and avoided publicity. He streamlined the NATO nuclear strike request system, quietly used his personal influence to help calm one of the Turkish-Greek crises, and generally worked to strengthen NATO. When a staff officer came to him with a proposal that "Lem" did not think was well thought out, he often rebuked him with ". .. you're calling from the jail. " The general retired in 1969.
A father of two, Lemnitzer-along with his wife of over 40 years, Katherine Mead Tryon-often gave Christmas parties for the children of his staff.