He attended the medical school of Saragossa where he graduated in 1873. He also became a Doctor of Medicine in Madrid in 1883.
He was the director of the Saragossa Museum (1879). He became a university professor at Valencia (1881), at Barcelona (1886), and at Madrid (1892). He was Director of the National Institute of Hygiene (1899). He founded the Laboratorio de Investigaciones Biológicas (1902) which later became the Cajal Institute (1922).
Among his many distinctions and memberships of societies, he was also made an honorary Doctor of Medicine of the Universities of Cambridge and Würzburg, and Doctor of Philosophy of the Clark University.
He published over 100 scientific works and articles in French, Spanish and German. His most famous works are "Rules and advices on scientific investigation," "Histology," "Degeneration and regeneration of the nervous system," "Manual of normal histology and micrographic technique," "Elements of histology, etc.," "Manual of general pathological anatomy," "New ideas on the fine anatomy of the nerve centres," "Textbook on the nervous system of man and the vertebrates," and "The retina of vertebrates."
His most famous studies were on the structure of the cortex of the brain. He discovered that the nervous system is made up of billions of separate nerve cells (neurons) and that nerve cells are polarizedized. He described the terminal branching of neurons, devised a way to stain nerve tissues, and made many other discoveries in the structure of the nervous system. For this work he was awarded the Nobel Prize in 1906, which he shared with Camillo Golgi.
He married Silvería Fañanás García in 1879 with whom he had four daughters and three sons. He died in Madrid on October 18, 1934.