"Ninety-nine percent" of the collection -- which included some 1,300 letters and manuscripts, and more than 500 paintings -- sold during the two-day auction, Osenat said.
"Forty lots were purchased or pre-empted" by museums, including the Musee d'Orsay in Paris, the neighbouring National Museum of the Legion of Honour and the National Archives.
The French government has a "right of pre-emption" enabling it to purchase items that it deems should remain in France.
The certificate from Napoleon Bonaparte's secret religious wedding to Josephine in 1804 sold for 32,500 euros ($35,700).
The document is signed and sealed by Cardinal Joseph Fesch, who presided over the clandestine wedding that took place at the behest of Pope Pius VII as a condition for his presence at Napoleon's grandiose coronation.
Among other notable pieces up for sale included a painting of the Empress Eugenie surrounded by her ladies in waiting, which sold for 161,400 euros.
The two large portraits of Napoleon III and the empress by German painter Franz Xaver Winterhalter went for 96,205 euros.
The personal belongings of the imperial family were also up for auction, with court dress worn by Napoleon III selling for 9,756 euros.
Forbes, a frequent presence at exclusive auctions, could have sold the collection in New York or London, but instead chose the Osenat auction house which specialises in France's First and Second Empires.
Osenat is based in Fontainebleau, south of Paris, the seat of French monarchs from the medieval Louis VII to Napoleon III, who reigned from 1852 to 1870.
A total estimated value of the collection and individual items has not been given.
Forbes, who maintains a chateau in Normandy and founded the American Friends of the Louvre, developed his passion for Napoleon III after his father offered him a portrait of the emperor when he was 16.