From 1907 to 1914, he studied in Berlin, Vienna, and Munich until finishing his doctorate, Carlo Rainaldi: Ein Beitrag zur Geschichte des Barocks, with Heinrich Wölfflin.
Hempel worked at the Albertina in Vienna until 1931, when he was appointed an associate professor at the University of Graz.
In 1933, Hempel became a full professor at Dresden Technical University.
[2] In November 1933, he signed the German Professors' Pledge of Commitment to Adolf Hitler and was charged with the managing of Cornelius Gurlitt's collection.
At this time, he also joined Der Stahlhelm, though his contingent was merged into the Sturmabteilung in 1934.