Initially trained as a bricklayer and architect, after serving in World War I Karl Bauer retrained as a bookseller and in 1926 joined the Callwey publishing house. He took control of the publishing house in 1930 following the death of Georg Callwey and went on to take seats in the German Publishers and Booksellers Association as well as the Booksellers Association. During the denazification, Baur was temporarily banned from working in the book industry but later re-established the Callwey publishing house, handing control to his wife and son in 1976.