Domenico Mariani was born in Posta, Italy. He was educated at the Lyceum "Angelo Mai", Rome, and the Pontifical Roman Seminary there. He was ordained on 18 December 1886 and performed pastoral work in Diocese of Rome until 1900. He was Canon of the Vatican Basilica until 1917 and was created Privy Chamberlain of His Holiness on 30 September 1914. He served as Secretary of the Cardinalitial Commission for the Administration of the Properties of the Holy See from 1917. He was raised to the rank of Domestic Prelate of His Holiness on 26 January 1917.
