In 1921, he moved to Ljubljana, in what was then the Kingdom of Serbs, Croats and Slovenes, and became the chief librarian of the National Research Library.

In the 1920s and 1930s, Pirjevec published several treatises on Slovene literature in the 19th century, especially the Romantic circle of France Prešeren and Matija Čop.

Together with Ivan Prijatelj and France Kidrič, Pirjevec was the main exponent of the positivist group of Slovenian literary historians of the interwar period.

Because of the anti-Fascist activities of his children, Pirjevec was arrested by the Nazi authorities of the Province of Ljubljana and sent to the Mauthausen-Gusen concentration camp in autumn 1943.

After the war, a bust of Pirjevec was erected in the main entrance hall of the National and University Library of Slovenia.