Corner joined the Ministry of Foreign Affairs in 1943, being recruited as one of its earliest foreign policy officers by then Secretary Alister McIntosh, who was in the process of building a professional foreign service. In his first years of service, he was seconded to the staff of H. V. Evatt, Australia's outspoken Foreign Minister. He was closely involved in drafting Evatt's speeches which defined an era known as "small power rampant". On his return he worked closely with Prime Minister Peter Fraser as he developed a more distinctly New Zealand position within the framework of the Commonwealth. He was New Zealand's political officer (policy adviser) at the Paris Peace Talks.