While working at the psychiatric clinic, Maria Montessori began a relationship with Giuseppe Montesano, a fellow doctor. Their relationship was kept private due to societal norms.

Maria Montessori had a son, Mario Montessori, out of wedlock with Giuseppe Montesano. Due to societal pressures and Maria's dedication to her career, Mario was initially raised by a family in the countryside. The relationship with Montesano ended soon after Mario's birth.

Around the time Maria Montessori achieved greater international recognition, she formally introduced Mario as her son and he became an integral part of her work, collaborating on Montessori projects and teacher training.

Maria and Mario Montessori shared a very close and collaborative relationship. He became her devoted assistant and helped promote and defend her work. They were nearly inseparable during their later travels and work together.

Mario Montessori later had children of his own, thus Maria became a grand mother in the last years of her life.