Mercier's grandfather Stanislas Mercier, a Protestant republican from Doubs, left metropolitan France and established himself in Algeria, then a French colony. His father, Jean Ernest Mercier, had served as military-interpreter of Arabic to the colonial army and was a prolific author. Ernest Mercier Jr., was the third son of five children. After studying at the École Polytechnique, he chose a career in the French Navy. He was posted to the port in Toulon, where he was responsible for modernizing the site, notably the electrical network. He completed his education at the École Supérieure d’Electricité between 1905 and 1908, during which time he married Madeleine Tassin (1881-1924), the daughter of a republican Senator. He was later noticed by Albert Petsche, and left the public sector for private electrical enterprise.