Hackl was born in Marburg an der Drau. At that time the town was part of Austria. Today it is part of Slovenia. His father was a surgeon. He learned anatomy from his father. He then studied art in Vienna. In 1865 he moved to Munich and studied at the Munich Academy of Fine Arts. He became a professor of drawing at the academy in 1880 and taught there until 1919. Hackl was a strict teacher. He wanted his students' drawings to be anatomically accurate. They learned by drawing plaster casts of ancient sculptures. His own drawings were very skillful and detailed.