Giovanni's father, a veterinarian, was a close friend of the Redemptorists of Madonna dell'Olmo, Montecchio Emilia, and after the closure of the convent in 1859, a sizable portion of its library was placed in the Mercati home.

During this period in Rome, Giovanni frequented the public sessions of Accademia di Conferenze storico-giuridiche, and was admitted to use of the Vatican Library in February 1890.

Have gained his doctorate, Mercati then underwent his obligatory military service in Florence as a medical attendant (soldato di sanità) until 1893.

During the early years of World War II, the Cardinal, a known opponent of Fascism, was protected and supported by a number of émigré scholars from Germany.

A prolific writer and great humanist, he understood Aramaic and the intricacies of racing automobiles and rocketry; he was even called the "most learned prelate to be elevated to the sacred purple" in a century.