Baptized in the church of Santa Maria in Portico, he studied at the Pontifical Roman Seminary, obtaining doctorates in theology and canon law) alongside Eugenio Pacelli, the future Pope Pius XII.

While doing pastoral work in Rome until 1908, he was named a professor at the "Institutions of Civil Law" of the Pontifical Roman Athenaeum S. Apollinare on 18 November 1904.

Raised to the rank of Privy Chamberlain of His Holiness on 18 May 1911, Massimi was made auditor (29 November 1915), pro-dean (19 February 1924), and dean (1 May 1926) of the Roman Rota.

On 21 September 1932, Massimi was appointed president of the commission that was to draw up a project of law on the judicial and procedural regulations of the tribunal of Vatican City.

Pius XII made him prefect of the Apostolic Signatura, the highest judicial authority in the church below the Pope himself, on 29 May 1946.