Walter Frodl obtained his doctorate in art history and archaeology in July 1930 from the University of Graz and worked thereafter as an unpaid assistant in the Kärntner Landesdenkmalamt (Carinthian Provincial Monuments Authority). After studying architecture for a few semesters at the Technische Hochschule (Technical College) in Graz, he was employed in the Klagenfurt branch of the Österreichisches Verkehrsbüro. He joined the NSDAP in May 1933. He continued his work as a research assistant in the field of monument preservation in December 1936 and in April 1937 was assigned to the Landeskonservatorat (provincial monument preservation authority) in Klagenfurt as a non-permanent public officer of the Zentralstelle für Denkmalschutz (Central Monument Protection Authority) and became its director a year later. Because of the large number of applications for export and the associated examination of art and cultural assets, not least as a consequence of the persecution of Jews by the Nazis, he was seconded in July 1938 to the export department of the Central Authority in Vienna and for a time also shared responsibility for the Landeskonservatorat in Salzburg. Two years later he was employed by the Kulturkommission (Culture Commission) in South Tyrol on behalf of the SS research institute Deutsches Ahnenerbe e. V., where he was responsible for registering German cultural assets. From winter 1941 he was a monument preservation consultant for Fritz Todt, Generalinspektor für das deutsche Straßenwesen (Inspector General for German Highways), during the construction of the Reich autobahns. After the restructuring and renaming in 1940 of the Central Monument Protection Authority within the Ministerium für innere und kulturelle Angelegenheiten (Ministry of Internal and Cultural Affairs) as Institut für Denkmalpflege (Institute for Monument Preservation) within the Reichsministerium für Wissenschaft, Erziehung und Volksbildung (Reich Ministry of Science, Education and Culture) in Berlin, the Gau monument protection authorities were incorporated in the respective Reichsstatthalterei (Reich governor's office). Frodl worked at the same time as Gau conservation officer for Carinthia. In February 1942 Friedrich Rainer, Gauleiter and Reichsstatthalter in Carinthia, appointed him director of the Gau museum in Klagenfurt and a public officer with tenure. At the end of 1942 he obtained his habilitation and became a lecturer in art history and monument preservation at the University of Graz and lecturer in monument preservation at the Technische Hochschule in the same city. From September 1943 until the end of the war he was responsible for monument protection in the Operational Zone of the Adriatic Littoral.

In 1946 the Carinthian provincial government dismissed Frodl from the public service as an "illegal" under § 14 of the Prohibition Act but revoked the dismissal retroactively in June 1947 but nonetheless removed Frodl from the service of the Carinthian provincial government. According to a certificate from the Klagenfurt city council of 25 July 1947, Frodl was classified as a minor offender. At the same time his habilitation was also revoked. He was supported, however, by Otto Demus in his application for re‑employment by the monuments authority. He worked initially as a contract employee in the restitution department of the Bundesdenkmalamt (Federal Monuments Authority) until he became Landeskonservator (provincial monument protection officer) in Styria in autumn 1948. In that year his habilitation was recognized again and he became a lecturer in monument protection again at the Technische Hochschule in Graz. He was allowed to teach at the University of Graz a year later. In 1952 he became head of the Institut für Österreichische Kunstforschung (Institute for Austrian Art Research) in the Federal Monuments Authority. In the same year he was appointed Staatskonservator 2nd class and two years later Staatskonservator 1st class. In 1956 he was authorized to teach at the Technische Hochschule in Vienna and was named an extraordinary professor in 1958. In subsequent years he continued his academic career in art history, monument protection and building documentation at various foreign universities. Apart from his functions on the advisory board for urban planning in Vienna and as a member of the examination board for higher civil servants (Höherer Technischer Dienst), he was also president of the Federal Monuments Authority from 1965 to 1970. In October 1970 he was awarded the Großes Ehrenzeichen für Verdienste um die Republik Österreich (Grand Decoration of Honour for Services to the Republic of Austria).