Giustini was named Prefect of Studies at the Athenaeum on 26 February 1878, a Privy Chamberlain of His Holiness on 11 December 1886 and a canon of Santa Maria in Trastevere on 21 May 1891.

He was made a Domestic Prelate of His Holiness on 18 August 1896, auditor of the Roman Rota on 11 February 1897 and later a judge of the Vatican Tribunal of First Instance.

Giustini was one of the cardinal electors who participated in the 1914 papal conclave, which selected Pope Benedict XV, who named him Prefect of the Discipline of the Sacraments on 14 October that year.

He also served as a member of the Commission for the Authentic Interpretation of the Code of Canon Law, and as Protector of the Order of the Friars Minor, more commonly known as the Franciscans.

After an exposition and funeral Mass at the church of Santa Maria Nuova, he was buried in the crypt of the Sacred Congregation of Propaganda Fide at the Campo Verano cemetery.