Medievalist. Ladner was the son of wealthy and cultured Jewish parents of Bohemian decent. His father Oscar Leopold Ladner, was a Viennese factory owner; his mother was Alice Burian (Ladner) (d. 1936). His family vacationed with among others, Sigmund Freud's and he knew Anna Freud well. Ladner attended the Bundesgymnasium XIX in Vienna, graduating in 1924. He early fell under the spell of the poet Stefan George and contemplated a career as a poet. He began studying history, archaeology and general history in 1929 under the so-called Vienna School art historians Julius Alwin von Schlosser and Josef Rudolf Thomas Strzygowski and Karl Maria Swoboda. At the same time, he studied diplomatics exegesis with the medievalist Hans Hirsch (1878-1940) at the Austrian Institute for Historicial Studies, taking a position the same year at the Monumenta Germaniae Historica, working under its director, the medievalist church historian Paul Fridolin Kehr (1860-1944). Ladner wrote his thesis under Schlosser, Die italienische Malerei im 11. Jahrhunderts, accepted for his Ph.D., in 1930 and published in 1931 and converted from a non-practicing Jew to Roman Catholicism. His first post-doctoral position was in the employ of the Kunsthistorisches Museum, under August Oktav Loehr, cataloging portrait medals. In 1934 he entered the Österreichischen Historischen Institut in Rom (Austrian Institute in Rome), where he began his research project on papal portrait iconography in the middle ages. Ladner published his habilitation in 1938, warmly acknowledging Hirsch, on medieval church reform, assuming a privatdozent position at the university in medieval history. With the annexation of Austria in March by Hitler, Ladner had his venia legendi (permission to lecture) withdrawn because he was "non-Aryan." He fled the Third Reich the same year to London making contact with the Warburg Institute. He moved to Canada to teach early Christian and medieval art history and history at the Pontifical Institute in Toronto. He rose to assistant professor of medieval history and archaeology in 1940 at the University of Toronto. The first volume of his study of papal portraits, begun in 1934, appeared in 1941. Between 1943 and 1946 he fulfilled Canadian war service in World War II in army intelligence. During this time he married Jocelyn Mary Plummer in 1942 (d. 1980). After the war he resettled in the United States, initially teaching at Notre Dame University in Indiana as assistant and then associate professor. He moved to Howard University in Washgington, D. C. as full professor, 1951, and then Fordham University in 1952. He was a member of the Institute for Advanced Study in Princeton, N. J., 1960-1961, following an initial association in the 1950s. His 1962 Wimmer lecture at UCLA summarized his research on image representation. It was published as Ad Imaginem Dei: The Image of Man in Medieval Art in 1965. In 1963 he accepted a position as professor of art history at the University of California, Los Angeles (UCLA), where he remained until his emeritus confirmation in 1974.
Ladner's early writing shows him engaged in Geistesgeschichte, or the history of spiritual and intellectual ideas, evident in his first articles in 1931 on the history of ideas. His conversion to Roman Catholicism drove his two great research projects, the iconography of the medieval popes and the idea of reform in the middle ages (Speculum). His interdisciplinarity, especially seeing art history in terms of world and cultural history, using a variety of research techniques, is a direct result of Vienna School art historians of the 1930s. Ladner is best known for his Idea of Reform: its Impact on Christian Thought, a "study of the ways and means employed in medieval civilization to reform culture and politics, both sacred and profane, within an understood sacred order" (Van Engen, Viator).