Errera's father was Italian; his mother was of German origin.
Léo Errera was born on September 4, 1858, in Laeken, Belgium. He was the son of a distinguished Venetian banker, Giacomo Errera, and of Marie Oppenheim. His father, who was the Italian consul general in Belgium, was an ardent patriot who had fiercely defended Venice against the Austrians in 1849. His maternal grandfather was a revolutionary who fought for new political ideas in 1830 in Frankfurt.