Korthals was sympathetic with the Dutch resistance against the German occupiers and worked as a journalist and editor for the underground newspaper Het Dagelijks Nieuws from July 1940 until May 1945.

Korthals was selected as a Member of the European Coal and Steel Community Parliament and dual served in those positions, taking office on 10 September 1952.

Korthals was selected as a first Member of the European Parliament and as the Delegation leader and dual served in those positions, taking office on 1 January 1958.

After the Leader of the People's Party for Freedom and Democracy and Parliamentary leader of the People's Party for Freedom and Democracy in the House of Representatives Pieter Oud announced his retirement from national politics and that he wouldn't stand for the election of 1963, he approached Korthals as a candidate to succeed him, but the People's Party for Freedom and Democracy leadership favored Minister of the Interior Edzo Toxopeus, in response Korthals withdrew his name for consideration and Toxopeus became the new Leader.

Korthals remained in active in national politics, in March 1964 he was nominated as a Member of the Council of State, taking office on 1 April 1964.