Pope John XXIII named him Cardinal Secretary of State and, in this position the most prominent member of the Roman Curia in Vatican City.

He attended the Angelo Braschi School and entered the Pontifical Roman Seminary in 1903 from which he graduated with honours in philosophy and theology.

After his election as Pope Pius XII, Pacelli appointed Luigi Maglione as his successor as Cardinal Secretary of State.

Maglione did not exercise the influence of his predecessor, who as Pope continued his close relation with Monsignori Giovanni Battista Montini and Tardini.

[2] Tardini and Montini continued serving there until 1952, when Pius XII decided to elevate both of them into the College of Cardinals[3] an honour which both turned down.

[6] Instead, Tardini was named Secretary of State by the newly elected Pope John XXIII, filling the previous vacancy.

Having declined being made Cardinal by Pius XII in 1953, he accepted the red hat in the consistory of 15 December 1958 with the title of Cardinal-Deacon of S. Apollinare alle Terme Neroniane-Alessandrine.