Born near Genoa, Giani was trained first in Pavia then in Bologna, where he studied with Domenico Pedrini and Ubaldo Gandolfi. In 1780 he moved to Rome, studying with Pompeo Batoni, Giuseppe Antolini and Christoph Unterberger at the Accademia di San Luca and beginning his independent career as a fresco painter. He absorbed influences ranging from ancient Greek and Roman art to Michelangelo's and Raphael's Renaissance classicism and the Romantic classicism of contemporaries such as Johann Heinrich Fuseli. Between 1784 and 1794 he lived in Bologna and Faenza, executing decorative commissions. In 1803 he was summoned to Paris. Until the fall of Napoleon, Giani divided his time between Paris and Italy; then he successfully survived the political changes and continued to execute decorative schemes in Rome. Giani worked in a distinctive Neo-classical style, creating sumptuous, richly coloured rooms, the paintings on walls and ceilings being surrounded with a wealth of antique ornament. He was a prodigiously talented draughtsman, who drew constantly, both out of doors and in the studio.