Whilst establishing himself as a notable Caravaggio scholar, Longhi retained a lively interest in Piero della Francesca, editing a monograph in 1928, representing him as the leading painter of the Quattrocento.

However, his first-hand viewing of many works, like those in the Borghese Gallery in Rome, led to the rediscovery of many lost masterpieces such as two panels of a Giotto altarpiece.

Longhi also rekindled interest in a large number of followers of Caravaggio, such as Hendrick ter Brugghen (he edited a monograph in 1927) and some painters from Ferrara.

During the Second World War, Longhi advised Eugenio Ventura, a dealer in Florence, who was investigated for his involvement in an exchange of pictures confiscated by the Nazi looting organisation known as the ERR or Reichsleiter Rosenberg Taskforce.

[6][7] In 1950, Longhi co-founded and edited with his wife Anna Banti Paragone, a bi-monthly magazine on art and literature still running to this day.