Sink attended Duke University (then known as Trinity College) for one year before securing an appointment to the United States Military Academy.

He became one of the four percent of the army's paratroopers qualified as a master parachutist and celebrated his birthday each year by making another jump.

He entered the National War College at Fort Lesley J. McNair in Washington, D.C. in August 1948, graduating in June 1949.

In October 1954, Sink was assigned to the Joint Airborne Troop Board at Fort Bragg, North Carolina.

In early 1955, he was transferred to Rio de Janeiro, Brazil, and in April 1955 assumed the dual functions of chairman of the United States Delegation to the Joint Brazil-United States Military Commission and chief of army section, Military Assistance Advisory Group, Brazil.

Sink returned to the United States and assumed command of the XVIII Airborne Corps and Fort Bragg in May 1957.