His father was a major surgeon in the 2nd horse regiment in Metz (Moselle). Frébault studied medicine in Paris and, after passing the doctorate examinations in 1849, practised his profession in the Gros-Caillou district. Residing at 237, rue Saint-Dominique-Saint-Germain (in the 7th arrondissement), he was appointed Surgeon Major of the 15th Battalion of the National Guard of the Seine during the First Siege, and also ran three ambulances; having retained his post during the Paris Commune, he was arrested by the Versaillais and immediately released.

A republican, in 1871 and again in 1874 he stood for election as a Paris city councillor, where he represented his district until 1876, when he ran for deputy in the 7th arrondissement and was re-elected until 1889. He spoke out in favour of free, compulsory and secular education, the abolition of the church budget and three-year military service.