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.
[1]: 255 After the war, he obtained in 1920 a doctorate from the University of Bucharest with a thesis about the Alexander romance in Romanian literature (Alexandria în literatura românească.
Cartojan published his thesis in two issues of the Yearbook of the Ioan Maiorescu Gymnasium of Giurgiu, and then more formally with Cartea Românească in 1922.
Its erudition, sound philological method and meticulousness allowed him to synthesize a vast set of materials dealing with folk books.
He joined the Medieval Academy of America in 1929, worked with Paul Van Tieghem on Répertoire chronologique des littératures modernes and was granted an honorary doctorate by the University of Padua in 1942.