After graduation, with a thesis entitled Some Words Regarding the Sensitization of Bacteria, she worked as an assistant at the clinical analysis laboratory that served Lisbon's public hospitals.

Between 1918 and 1948 she was head of the Physiotherapy Services in public hospitals in Lisbon, also working as a general doctor and a high-school teacher.

[1][2] As a feminist and pacifist, she was involved with Portuguese and international pacifist groups, in addition to being a co-founder of the Portuguese Group of Feminist Studies [pt] (Grupo Português de Estudos Feministas), which was formed in 1907, led by Ana de Castro Osório, Adelaide Cabete, also a doctor with whom Quintino had studied in Lisbon, and Maria Veleda.

"Assistance of the Portuguese to the Victims of War" was formed by the Catholic Church but, arguing that such care should not be associated with religion, Sofia Quintino was one of the major drivers of Pela Pátria, a secular organization created in 1914, that conducted the first nursing courses in Portugal that were not held just for nuns.

After Germany declared war on Portugal in March 1916, Quintino was head of nursing training of the Portuguese Women's Crusade, which provided assistance to the mobilized soldiers and was one of the first institutions in Portugal to organize women for the war effort, carrying out activities such as making warm clothes that were sent to the front.