 Marella was born in Rome to Luigi and Vincenza (née Baldoni) Marella, and studied at the Pontifical Roman Seminary and the La Sapienza University.He was ordained to the priesthood by Cardinal Basilio Pompili on 23 February 1918, and then furthered his studies whilst doing pastoral work in Rome until 1922.From 1922 to 1924, Marella was an official of the Sacred Congregation for the Propagation of the Faith in the Roman Curia.