Carstens was born in the City of Bremen, the son of a commercial school teacher, who had been killed at the Western Front of World War I shortly before his birth.

He studied law and political science at the universities of Frankfurt, Dijon, Munich, Königsberg, and Hamburg from 1933 to 1936, gaining a doctorate in 1938 and taking the Second Staatsexamen degree in 1939.

In 1940 he joined the Nazi Party; reportedly, he had applied for admission in 1937 to avoid detrimental treatment when he worked as a law clerk.

In July 1960 Carstens reached the position of secretary of state at the Foreign Office and in the same year was also appointed as professor for public and international law at University of Cologne.

During the grand coalition government of 1966-1969 under Chancellor Kurt Georg Kiesinger, he first served as secretary of state in the Ministry of Defence, and after 1968 as head of the German Chancellery.

He also famously denounced the author and Nobel laureate Heinrich Böll as a supporter of left-wing terrorism (specifically, the Baader-Meinhof Gang) for his 1974 novel The Lost Honour of Katharina Blum.