 His father, Francois Marie Benoist, and his mother's father, Charles Sanguinet, both arose from prominent families and were among those individuals who were instrumental in laying the foundations for St. Louis.After devoting two years to the study of medicine under one of the pioneer physicians of the city, Benoist undertook the study of law in the office of Horatio Cozzens and became licensed to practice in the legal profession.Thereafter, he entered into practice in partnership with well-known attorney and conveyancer, Pierre Provenchere.