His father, the general Alexandre, Viscount de Beauharnais, was guillotined on June 23, 1794. The marriage of the general's widow, Joséphine Tascher de La Pagerie, to Napoleon Bonaparte on March 9, 1796, was at first resented by Eugène and his sister Hortense, but their stepfather proved kind and genuinely interested in their welfare. Eugène in turn was a useful military aide to Napoleon, particularly in the coup d'état of 18 Brumaire (Nov. 9, 1799) and the victory over the Austrians at Marengo (June 14, 1800). In 1804 Eugène received the title of prince and was appointed archchancellor of state.

In 1805, when Napoleon proclaimed himself king of Italy, Eugène became his viceroy there. He reorganized public finances and the civil service, built roads, and introduced the French legal system.

In the war against Austria in 1809, Eugène, as commander of the Italian army, won an important victory at Raab (Györ) and fought well at Wagram. He also distinguished himself in Russia in 1812 and in Germany the following year. In 1814 he held out as long as possible in Italy against the Austrians and the Neapolitans, resisting their attempts to induce him to desert Napoleon. Finally, however, he had to conclude the armistice of Schiarino-Rizzino (April 16, 1814). He then retired to Munich, to the court of the Bavarian king Maximilian I, whose daughter Amelia Augusta he had married in 1806 and who gave Eugène the title of Duke von Leuchtenberg.