Born in Babenhausen, Germany, Hoerman worked in a Hamburg shipyard before emigrating to the United States in 1904. Settling in Chicago, he studied architecture and opened an architectural office in 1909. By 1920, Hoerman was devoting most of his time to painting, and he opened a studio and gallery in the resort town of Saugatuck, Michigan. There he became known as a painter of the Lake Michigan shoreline and sand dunes.
Hoerman probably was a self-taught artist. He painted in a realistic manner with slightly simplified drawing and vivid color.
In 1927, Hoerman and his artist wife, Christiana Ackermann, began making trips to the Southwest where they painted desert and mountain scenes and, most especially, the Grand Canyon. They also enjoyed painting other exotic landscape and genre scenes from their travels to Europe, Mexico, and North Africa. They may have maintained their home in Michigan throughout their lives, but they also had residences in Palm Springs and Riverside, California, and are most associated with the West in their later years.