Jager, the son of a railroad signalman, was born near the town of Vrhnika. He went to study architecture in Vienna, where the famed architect and urban planner Otto Wager was one of his professors. Jager socialized with many prominent Slovenians studying and working in Vienna, including architects Maks Fabiani and Jože Plečnik, both of whom were at the forefront of the latest architectural trends.

As a young architect, he worked on several innovative projects, including a café in Ljubljana, which he decorated with motifs from Slovenian folk design.

Unlike more traditional architects, Jager was intrigued by skyscrapers and other new developments emanating from the United States. In 1901, he immigrated to Minneapolis, where he quickly established himself in architectural circles. Just two years after his arrival, he penned an influential article calling for a more original approach to American church architecture. Jager was intrigued by the innovations developed by Louis Sullivan and the Chicago School of Architecture, which rejected traditional European-style designs.

During World War I, Jager was sent by the U.S. government to Europe, where he helped to rebuild houses in Greece. After his return to Minnesota, he became known as the Builder of Minneapolis – not because of his individual projects, but because of his contributions to the innovative designs by two local architectural offices.

For years, he worked for the architectural office of Hewitt and Brown. Known as the “quiet partner,” he was a key person behind several major projects in the Upper Midwest. He was among the architects who drafted the urban plan for the city of Minneapolis, where he also designed a city park, as well as the tree-lined Red Cedar Lane. Still known as one of the most pleasant streets in the city, Red Cedar Lane attracts architectural connoisseurs who come to admire both the street itself and the homes on it, several of them designed by Jager.

Jager became known for his writings in architectural publications, in which he urged his colleagues to incorporate nature in their work and to adapt architecture to the environment – a novel concept at the time.

Jager died in 1959, but his ideas lived on, and remain one of the most important legacies of a Slovenian architect who shared his passion for architecture and the natural environment with the younger generations.