Born near Genoa in San Sebastiano in the middle of the 18th century, Giani was trained first in Pavia then in Bologna, where he studied with Domenico Pedrini and Ubaldo Gandolfi. In 1780, age 22,  he moved to Rome, studying with Pompeo Batoni, Giuseppe Antolini and Christoph Unterberger at the Accademia di San Luca to begin 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. He died in Rome in 1823.