Maurice's mother, Pauline Bauer, was a Hungarian Jew who adopted French nationality and converted to Catholicism during the Second Empire.

Weil joined in the military in the latter years of the Second Empire, and was incorporated into the 8th Battalion of the Garde Nationale Mobile when the corps was established in February 1868, and was promoted to second lieutenant in 1869.

In 1875, Weil joined the Ministry of War, where his reputation for proficiency in foreign languages, and in particular German, earned him an assignment to the "Statistical Section" (the official name for the intelligence service).

During this appointment, Weil became acquainted with Hubert-Joseph Henry and Ferdinand Walsin Esterhazy, who would both later become directly implicated in the Dreyfus Affair conspiracies.

In 1896, as efforts to revise to Dreyfus verdict were ongoing, Weil was notified that Deputy André Castelin would publicly denounce him as Esterhazy's accomplice in betraying military secrets to Germany.