Art historian of the so-called "New" or "Second" Vienna School group, known for his work on medieval and Baroque architecture. Sedlmayr studied architecture at Vienna's Technische Hochschule between 1918-1920. Thereafter he studied art history at the University of Vienna under Max Dvořák until Dvořák's death in 1921, and then under Dvořák's successor, Julius Alwin von Schlosser. His 1925 dissertation, written under Schlosser, was on the Austrian baroque architect Johann Bernhard Fischer von Erlach.

Specialist in ancient Greek art, particularly the art and architecture of Athens. Director of the Austrian Archaeological Institute (österreichisches archäologische Institut, öAI) in Athens, 1921-1938. Because of monetary restrictions, also served in diplomatic corps as Austrian Consul General in Athens 1923-1933. Named second secretary of the German Archaeological Institute in Athens in 1938 following the German takeover of Austria.