Georgios Iakovidis served as the Gallery’s supervisor until 1918. Under Liberal administration, Bill 1434 ‘On establishing the National Gallery’ came into force, in 1918 – it was the first effort to overhaul and improve the museum’s administrative and financial status. Among other things, the law abolished the position of a supervisor, which until then was held by Georgios Iakovidis, and instituted the post of a director, which was taken by the writer and art critic Zacharias Papantoniou. In addition, an annual grant from the ministry's budget was set up for collection enrichment, for the first time, and a Board for artistic matters was established to advise the Ministry on acquisitions and donation acceptance, in place of the supervisor and the supervisory committee. The first Art Board consisted of Georgios Iakovidis (chairman) and members Zacharias Papantoniou, Georgios Hatzopoulos, the writer Konstantinos Hatzopoulos and the painter Konstantinos Maleas. The same law instituted, also for the first time, the sculpture collection.

Papantoniou sought to resolve the Gallery’s major issues, unsuccessfully. He worked towards the merger of the National Gallery and the estate of lawyer and art connoisseur Alexandros Soutsos (1839-1895), who bequeathed his estate to the Greek state for the purpose of establishing and maintaining a museum of painting. In 1926, to resolve the housing issue, in cooperation with the Soutsos Estate Trustees, he acquired the Iliou Melathron, but cancelled the acquisition when he found out that the building was not suitable for a museum.

Acquisitions to enrich the collections using the annual subsidy by the ministry, or special state subsidies involved works mainly by Greek artists, including Nikolaos Doxaras, Nikolaos Kantounis, Nikolaos Gyzis, Nikephoros and Nikolaos Lytras, Konstantinos Maleas, Dimitris Galanis, Panos Aravantinos, Konstantinos Parthenis, Nikolaos Cheimonas, Periklis Pantazis and Vassilios Hatzis, so as to provide a representative overview of painting from the Ionian School to modern times.

In 1931, moreover, the National Gallery’s most important acquisition was made at auction in Munich: Domenikos Theotokopoulos’s Concert of the Angels, the upper part of his Annunciation, was acquired for the sum of 5,900,775 Drachmas.

Under Zacharias Papantoniou, the sculpture collection was also inaugurated. Thus, in 1934, the new permanent display of the painting collection featured sculptures for the first time. Papantoniou also opened the museum to the public on a daily basis, introducing visiting hours with free admission.