When he was a little past 30, Ferri completed the painting of the ceilings and other internal decorations begun by his master in the Pitti Palace, Florence. He also cooperated in or finished several other works by Pietro in both Florence and Rome. Of his own independent productions, the chief is an extensive series of scriptural frescoes in the Church of Santa Maria Maggiore in Bergamo and a painting, considered to be his best work, of St. Ambrose healing an invalid, which is the principal altarpiece in the Church of Sant' Ambrogio in Rome. He executed a large number of etchings and frontispieces for books.