Lake joined the State Department in 1962, serving until 1970 as a Foreign Service Officer. Lake was an assistant to Ambassador Henry Cabot Lodge Jr. during the Vietnam War. His State Department career included assignments as consul at the U.S. Embassy in Saigon, South Vietnam (1963), vice consul in Huế (1964–1965) and special assistant to the assistant to the president for national security affairs (1969–1970) in the Nixon administration. In 1969, he accompanied National Security Advisor Henry Kissinger on his first secret meeting with North Vietnamese negotiators in Paris. In 1970, he had a falling-out with Kissinger over the Nixon administration's Cambodian Campaign and left the State Department as a result. He later wrote a book critical of Kissinger's approach to Africa.