Before his appointment to the emperor's cabinet in 1806, Fain had worked since 1795 in various state archives. Napoleon made him a baron in 1809 and in 1813 appointed him his private secretary. During the Hundred Days in 1815 he resumed this post but then retired to private life. In 1830 King Louis-Philippe appointed him occasional administrator of the civil list and first secretary of his cabinet.

During his retirement Fain published his reminiscences, most notably Manuscrit de 1814, contenant l'histoire des six derniers mois du règne de Napoléon (1823; “Notebook of 1814, Containing the History of the Last Six Months of Napoleon's Reign”) and similar “Notebooks” for 1813 and 1814, as well as for 1794–95 (the Revolutionary year III). Fain's memoirs were published posthumously in 1908.