The Brentano family, of Italian (Lombard) origin, had settled in the Landgraviate of Hesse-Darmstadt in the 17th century and were recognized as Hessian nobles, with close contact to important figures of the German Romanticism, including Goethe, Savigny and Arnim.

He also attended the proceedings of the Parlamentarischer Rat assembly drafting the Basic Law for the Federal Republic of Germany.

In the federal elections of 1949, he obtained a seat in the West German Bundestag parliament by directly winning the mandate of the Bergstraße constituency.

In 1952 he and other CDU MPs advocated the implementation of a plurality voting system instead of the personalized proportional representation concept, though to no avail.

After the Allied occupation statute had been lifted in 1955, he was appointed Foreign Minister of Germany at the suggestion of Chancellor Konrad Adenauer, who prior had filled the office himself.