When he was still a baby, he moved to Pontevedra with his family, where he lived until he was fifteen. On 3rd May 1710, he moved to Madrid and entered the Benedictine Order. In 1714, he went to Salamanca to study Theology. It is then that he changed his name and registered at University as Fray Martín Sarmiento. After a brief stay in Asturias, he returned to Madrid but he went to Pontevedra first to say good-bye to his mother (this summer trip was reflected in many of his works). He lived in Madrid for twenty years but during this period he stayed in Toledo for more than a year. These are his most important works of that time: Martinus contra Martinum (1726), Demostración crítico-apologética del Theatro Crítico Universal (1732), Memorias para la historia de la poesía y poetas españoles (between 1741 and 1745) and Reflexiones literarias para una Biblioteca Real (1743). The year 1751 is extremely important in his scientific activity. His interest in Natural History and Botany began to develop and was shown in his work Pensamientos Crítico-Botánicos (1753-62).
The Royal Botanical Garden was created at the request of Father Sarmiento. Since 1743, he demanded the creation of public libraries and botanical gardens in several cities in the style of the European Gardens as well as the constitution of the Academy of Agriculture and the professorships of Natural History, Agriculture and Botany. His visits to Galicia had an influence in his work Coloquio de 24 gallegos rústicos. In 1955, he returned to Madrid and started his work Catálogo de voces vulgares, y en especial de voces gallegas de diferentes vegetables, and the second part of Catálogo de voces y frases de la lengua gallega. This is the most important period of his studies. He works on Botany to classify the materials collected in Galicia and began to write many works on this subject. His studies are considered as excellent catalogues of Galician linguistic materials of a time in which the elaboration of a Galician Dictionary was still impossible. Fray Martín Sarmiento always fought for his region. He focussed on the analysis of the Galician language, which was discredited at that time.
Ramón Mariño has collected the thesis on the work written by Father Sarmiento. He thinks that they centre on the linguistic, semantic and communication problems of Galician language as a result of the backwardness in Galicia. He rejected the physical punishment suffered by some children for speaking Galician at school. He defended that children should be taught the same language they spoke at home and parishioners preached in his own language. This implied the introduction of Galician language at school, court, church and Administration. The Galician Academy dedicated him the Day of the Galician Letters ('Día das Letras Galegas') in 2002.