Born in Uzunu, Giurgiu County, his parents were Anghel Cartojan and Maria ("née" Petrescu). He graduated from Bucharest's Saint Sava National College in 1902. He then enrolled in the literature and philosophy faculty of the University of Bucharest, where Ioan Bianu was one of his professors, and graduated in 1906. Early on, he developed an interest in early Romanian literature and in researching the manuscripts of the Romanian Academy Library, where he worked from 1906 to 1912. At the same time, he was a teaching assistant. From 1912 to 1914, Cartojan attended speciality courses at Friedrich Wilhelm University in Berlin. Upon returning to Bucharest, he was appointed principal of the Ioan Maiorescu Gymnasium. He married Sevastia Condeescu on April 14, 1916. After Romania entered World War I and the Central Powers occupied Bucharest in December 1916, the gymnasium was turned into a detention camp for Romanian and Russian prisoners of war. Cartojan continued teaching for a while at the Matei Basarab and Gheorghe Lazăr high schools; after being imprisoned for a short time by the occupying forces, he managed to reopen the gymnasium in Giurgiu.