Louis Bernheim was born into a Jewish family in Saint-Josse-ten-Noode, Brussels in Belgium on 1 September 1861. His parents had emigrated to Belgium from Nancy, France in 1858. Entering the École Militaire in 1878, he joined the Regiment of Grenadiers as a second lieutenant at the age of 19. He later taught at the Royal Military Academy in Brussels and rose through the ranks rapidly. By the time of the German invasion of Belgium in August 1914, Bernheim was serving as a lieutenant-colonel in the 7th Regiment of the Line.