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.

He received his episcopal consecration on the following 29 October from Cardinal Pietro Fumasoni Biondi, with Archbishops Carlo Salotti and Domenico Spolverini serving as co-consecrators, at the chapel of the Collegio de Propaganda Fide in Rome.

[citation needed] Marella succeeded Archbishop Angelo Roncalli (the future Pope John XXIII) as Nuncio to France on 15 April 1953.

[3] Pope John XXIII created him a cardinal at the consistory held on 14 December 1959 and he received his red biretta in Paris from President Charles de Gaulle.

[6] He acted as papal legate to the inauguration of the Vatican pavilion at the New York World's Fair presiding over the unveiling of the Pietà on 20 April 1964[7] Marella received four honorary doctorates on this visit to the United States, including one from the Catholic University of America, which had prohibited four liberal theologians from delivering lectures there the previous year, for which Marella commended the university.