The end of the Monarchy brings about changes in the organisation of the former Natural History Court Museum. To satisfy the
terms of the Peace Treaty of St. Germain, it becomes the property of the Austrian state. The board of management is dissolved;
the last director, Franz Steindachner, retires. The Museum of Natural History is attached to the State Office for Education,
later to be the Federal Ministry of Education. The managers of the five departments of the house are granted greater autonomy.
The program of the whole museum is supervised by a council having nine members. Berwerth, the director of the Mineralogical-Petrographical Department retires, dying shortly thereafter. The custodian Ferdinand Wachter, who had done research work in the Hohe Tauern (Central Eastern Alps) together with Berwerth, withdraws from active service for health reasons. Management of the department is taken over by Koechlin. He is also elected chairman of the Museum Council. Supporting him in the work of the department, from April 1919 onward is his assistant Hermann Michel. Both are also members of the Council of the Viennese Mineralogical Society (from 24 November 1947: Austrian Mineralogical Society). Furthermore, Michel is manager of the "Technische Untersuchungsanstalt für Edelsteine" (Technical Analysis Institute for precious stones) and a legally recognized expert for the Commercial Arbitration Court in Vienna. Koechlin becomes director of the Mineralogical-Petrographical Department, retiring with the grade of Hofrat in 1922 in which year Michel was appointed custodian.
The political turbulences of the times between the wars also have their impact on the activities of the department. Although this period is marked by an addition of about 12,000 objects to the collection, no major purchases are made - apart from the acquisition of the meteorite of Lanzenkirchen which had fallen on August 28, 1925, which could only be obtained with the support of the "Friends of the Museum of Natural History". It was not until 1932 that Alfred Schiener joins the department as a voluntary helper, becoming provisional scientific assistant in 1936.
Beginning in 1923, Hermann Michel first becomes manager, later in the same year director of the department and in 1933 head of the entire museum until the annexation of Austria by Germany. Despite financial limitations and administrative activity as managing director, Michel was at first able to intensify research activities in the department. Research on precious stones, initiated by him in 1938, is continued by Hubert Scholler after World War II to maintain the tradition. In 1938, after Austria was incorporated into the Third Reich, Michel is released from his position of managing director, but management of the Mineralogical-Petrographical Department remains in his hands. In the fall of 1938, Heinz Meixner becomes scientific assistant in the department, becoming custodian in 1940. Meixner is a keen, even fanatic mineral topographer. Attendance, maintenance and care of the mineral collection is thus assured.
The outbreak of World War II brought a slowdown in scientific activities and in particular in the display collection activities and with it the function of the Mineralogical-Petrographical Department, leading gradually to a complete halt. Already in the early summer of 1939 the meteorite collection and the gemstone collection are removed from the show rooms and packed in containers for air-raid protection. These measures are carried out to prevent the pilferage or illegal removal of valuable items from the museum. With the onset of war, large sections of the collection and the library are at first deposited in chambers on the ground floor, later in other more secure areas in Vienna and at Kirchstetten Castle near Staatz in Lower Austria. During the late fall of 1944, further sections of the display collection (i.e., minerals and rocks), as well as from the extensive storage, are moved to a salt mine at Lauffen near Ischl. Deposition operations are carried out under Michel's direction; there is no significant damage to material from the collections due to these activities. The mineral collection is thus one of the few in central Europe that have never been subject to any significant damage in times of trouble. This fortunate circumstance enhances its scientific and cultural-historical significance.
Meixner loses his position after the collapse of the national socialist regime and the restoration of Austria as an independent state. Michel resumes the position of managing director of the museum in 1947 and continues in this function until 1951. Hubert Scholler is appointed provisional scientific assistant and is given the grade of custodian 2nd class. A very important acquisition at this time is the transfer of substantial collection material (minerals, ores and rocks) from the Federal Geologic Survey to the department, which had previously been reserved from the doublets inventory of the Minerals Cabinet for the Imperial Royal Mining Museum, at that time at the planning stage. These had been put aside in accordance with the directives of the supervisory authority governing designated occurrences, among others, those minerals and rocks, which had earlier been assigned in 1844 from the doublets inventory of the Minerals Cabinet. There are also other substantial stocks from this institute assigned to the department; among these is the "Friese Collection".
The former collection of the Count von Breunner, which constitutes the basis of the k.k. Montanistische (Mining) Museum, the fore-runner of the k.k. Geologische Reichsanstalt (Geological Survey) is also transferred in this transaction. In addition there were many items shipped by mining officials of the Monarchy and co-workers of the Institute. Up to the middle of the 1960's, packaged materials stayed in boxes; after appraisal, large portions were added to complement the inventory of the department, where needed. During the years thereafter, doublets were used in exchange, thereby to a degree compensating for the budgetary short-fall, enabling the acquisition of further valuable material. By far the largest part of the specimens remain as doublets within the department.
Scholler is not entirely well. He avoids fieldwork, but instead assiduously clears the backlog of work on the older parts of the collection, which reached as far back as the end of the Monarchy and devotes the remainder of his time to gemmology, the museum archives, as well as popular education. Scholler continues the tradition of the examination of gems at the Museum, as established by Michel. This leads in 1954 to the founding of the State Gem Institute at the Natural History Museum, but which remains attached to the Mineralogical-Petrographical Department.
Schiener's particular task is the work with the mineral collection and his special field is mineralization within the Gastein area. He had already become the head of the department in 1949, becoming its director in 1953.
He dies suddenly on August 23, 1962. Management of the Department is transferred to Hubert Scholler. Within this time, work on modifying the systematic minerals and meteorite show collections was begun - assisted by Gero Kurat, who starts his service in the department in 1962. Scholler becomes director of the department in 1964. In January of the same year, Gerhard Niedermayr begins his service in the department. He continues with the classification of the Meteorite Show Collection and later with the Mineral Show Collection. The collection is rearranged in accordance with the Klockmann-Ramdohr system, using the organisation of Strunz for the new display.
After the retirement of Hubert Scholler, Karl Rechinger, director of the Botanical Department and concurrently managing director of the Museum of Natural History also provisionally directs the Mineralogical-Petrographical Department with the beginning of 1967. As from July 1, 1968 Gero Kurat is provisionally appointed to direct the department. The former director Hubert Scholler continues with his research work in gemmology until his death on April 27, 1968. During an absence abroad of Gero Kurat of almost one year (from November 1, 1970 to September 30, 1971), the present managing director and simultaneous director of the Department of Geology and Mineralogy, Friedrich Bachmayer, is provisionally in charge of the department. The financial and personnel situation within the Department shows a gradual improvement.
Walter Cadaj is active in the department from August 1, 1968. As an employee, Hans Klob is active in the Department from the beginning of 1970; he goes on leave as of September 1, 1971 and ceases to be on the personnel roster as of May 29, 1972. Robert Seemann, employee, takes on Klob's assignments as of September 1, 1971. Other scientific additions to the staff are Alfred Kracher (1977-1982), Franz Brandstätter (from 1 February 1982), Vera Hammer (from 1 April 1992), Uwe Kolitsch (from 1 January 2007), Ludovic Ferrière (from 1 February 2011), Julia Walter-Roszjár (from 1 February 2014) and Lidia Pittarello (from 2 September 2019). Working within the department on scientific research projects financed by the "Fonds zur Förderung der wissenschaftlichen Forschung" (FWF; Austrian Science Foundation) on a time-limited basis includes the following: Georg Hoinkes (1972-1974), Alfred Kracher (until 1977), Rainer Schultz-Güttler (1977), Franz Brandstätter (until 1982), Theodoros Ntaflos (until 1992), Thomas Presper (1992-1994), Jürgen Walter (1994-1995), Cecile Engrand (1996) and Aurore Hutzler (2015 bis 2017).
During the 1970s, important acquisitions, from new inland and foreign finds, partly through purchase and partly through exchange, could again be made in increasing measure. The display collection is modernised and electric illumination of the collection is gradually begun. A beginning is made with Hall IV, which is devoted to the newly displayed Gem Collection. The entire display collection is closed during the years 1973 to 1975 for renovation work. Halls I-III are reopened in 1976; the Hall of Gems (Hall IV) is ceremoniously reopened on November 8, 1977. Specially secure showcases now enable the viewing of the most valuable parts of the collection. The completely redesigned large wall display case on decorative and building stones in Hall I is opened in March 2015. A new large wall display case on the „Evolution of minerals“ in Hall I is presented to the public in April 2017. In January 2018, the completely redesigned gemstone display in Hall IV is opened.
Enormous improvements could be made in the instruments with which the Department is equipped. This includes the step-wise acquisition of a modern Zeiss microscope, an electron microprobe (1974), modern X-ray diffractometers (1975 and 1995), a scanning electron microscope (1990) with an energy-dispersive X-ray analysis auxiliary and a Leitz research microscope (1994), almost all with the financial support of the Austrian Research Fund FWF). The chemical laboratory and sample preparation equipment are also renovated. In the course of installing subterranean storage for the Museum (1991-1994), a new depository hall for the Department of Mineralogy and Petrography is constructed. An extension to the library, long overdue, is also completed (1996). In 2012, a modern scanning electron microscope (with energy-dispersive X-ray analysis, electron backscatter diffraction, cathodoluminescence and low-vacuum facilities) and a state-of-the-art electron microprobe could be purchased. These two instruments were later assigned to the newly created department "Central Research Laboratories."