There was lot of opposition from the men students in her class, they were hostile and furious. She had to do her practical alone. At home Maria had the full support of her mother.
At the end of the course she stood first in her class. She was the first woman to receive the degree of Doctor of Medicine of the University of Rome in 1896. She became an instructress in her University's psychiatric clinic. In the course of her work she happened to visit an asylum which housed mentally defective children. The sight and plight of this children compelled her to work with them. This work leads her to the conclusion that in training of the mentally deficient the best approach was pedagogical rather than medical. And these children passed the State Examination in reading and writing with percentage equal to healthy and happy children of ordinary children. She was wondering why the healthy and happy children (the so called normal children) were doing only at an average level.

She presented her report on her experiments to the Pedagogical Congress in Turin in 1898 and this resulted in starting special schools for the mentally defective children in Italy and other countries. In the same year she was appointed as the directress of SCUOLA Ortofrenica in Rome. She held the post for two years and this is when she first came into contact with the pedagogy and took interest in education. Shortly afterwards, she was chosen to represent Italy at two different women's conferences, in Berlin in 1896 and in London in 1900. Shifting her focus from the body to the mind, she returned to the university in 1901, this time to study psychology and philosophy. At the same time she was studying the method and regulations that was prevalent for normal children. In 1904, she was made a professor of anthropology at the University of Rome.

She was given an opportunity by the Istituto Romano dei Beni Stabili. She had to work with 50 children of age ranging from 3 to 6 years in one room whose parents were both working. The children were real urchins, committing petty crimes left on their own unattended. The outcome of this project was the first Casa dei Bambini or The House of Children. The children were under the care of Dr. Montessori and her assistant, the daughter of the caretaker. Lot of toys was purchased for the children by the bankers. The children initially used the toys but Dr. Montessori discovered their shortcomings and added Sequins apparatus, those that she had used with the mentally defective children, and Exercises of Practical Llife and Sensorial materials. Children showed interest in these and the toys were discarded. Montessori studied the use of the apparatus and she put them to test and those that passed were retained and the rest were discarded. She arranged the apparatus in psychological order depending on the usage by different age groups.

Children choose their work and repeated it to satisfy their inner needs and attain perfection. There was a difference in the behaviour and attitude of the children. For the first time they were treated as individuals. The scientifically designed, didactic, precise, and self corrective materials/apparatus was especially prepared for the children. Even the dullest child worked with it and recognized the challenge and was able to concentrate on his work and enjoyed his/her discovery at the culmination of the work. Then Dr. Montessori slowly introduced language on the request of the parents and also arithmetic. This ultimately became the Montessori method of education developed there, based upon Montessori's scientific observations of these children's almost effortless ability to absorb knowledge from their surroundings, as well as their tireless interest in manipulating materials.

Every piece of equipment, every exercise, every method Montessori developed was based on what she observed children to do "naturally, " by themselves, unassisted by adults. Children teach themselves. This simple but profound truth inspired Montessori's lifelong pursuit of educational reform, methodology, psychology, teaching, and teacher training—all based on her dedication to furthering the self-creating process of the child. Maria Montessori made her first visit to the United States in 1913, the same year that Alexander Graham Bell and his wife Mabel founded the Montessori Educational Association at their Washington, DC, home. Among her other strong American supporters were Thomas Edison and Helen Keller. In 1915, she attracted world attention with her "glass house" schoolroom exhibit at the Panama-Pacific International Exhibition in San Francisco. On this second U.S. visit, she also conducted a teacher training course and addressed the annual conventions of both the National Education Association and the International Kindergarten Union.

The committee that brought her to San Francisco included Margaret Wilson, daughter of U.S. President Woodrow Wilson. The Spanish government invited her to open a research institute in 1917. In 1919, she began a series of teacher training courses in London. In 1922, she was appointed a government inspector of schools in her native Italy, but because of her opposition to Mussolini's fascism, she was forced to leave Italy in 1934. She traveled to Barcelona, Spain, and was rescued there by a British cruiser in 1936, during the Spanish Civil War. In 1936 she was invited to India by the Theosophical Society, (Dr. Arundale and his wife Rukmani Devi Arundale). She opened the Montessori Training Centre in Laren, Netherlands, in 1938, and founded a series of teacher training courses in India in 1939.