Joseph Louis LeCornu was born in Caen, Normandy, on March 13, 1864, the seventh of eight children. His father was a lacemaker by trade. Following the father's death in 1878 when LeCornu was 14, he and his seven siblings were raised by their mother.
An early achiever, LeCornu received a prize in philosophy from his lycee in Caen at age 9 and four years later was included in a delegation from his school sent to Paris to attend the burial of writer Victor Hugo. The following year he was admitted to L'Ecole Centrales des Arts and Manufactures.
A good student if a bit unruly, LeCornu was disciplined once for causing a disturbance in the dining hall, a second time "for having taken part in a noisy demonstration against freshman."