Together with Václav Havel, Zdeněk Mlynář, and Pavel Kohout, Hájek was one of the founding members and architects of Charter 77.

From 1955 Hájek worked in diplomacy: in 1955–1958 as an ambassador in Britain, in 1958–1962 as a deputy of the minister of foreign affairs, and in 1962–1965 he represented Czechoslovakia in United Nations.

After the Soviet Union army took control over Czechoslovakia (21 August 1968) he protested against this in a speech at the United Nations (where he used the word occupation) – this caused his dismissal from high offices and even from the communist party (1970).

Together with Václav Havel, Zdeněk Mlynář, and Pavel Kohout, Hájek was one of the founding members and architects of Charter 77.

After the fall of socialism in Czechoslovakia (1989) Hájek served as an advisor of Alexander Dubček (1990–1992) but was unable to obtain significant political influence.