In 1905, he returned to the United States to work in Pittsburgh, Pennsylvania, for the architectural firm MacClure & Spahr.
He is best known for monumental buildings such as the Pittsburgh Athletic Association (1911), the Masonic Temple (1915 - now Alumni Hall of the University of Pittsburgh), William Penn Hotel (1916 and again in 1928), Mellon Institute (1937), the Longue Vue Club (1923), Rolling Rock Club and Stables (1928 - near Ligonier, Pennsylvania), the T.W.
The prevailing architectural motif of these Benno Janssen homes was a picturesquely irregular configuration of buildings rambling around a central courtyard.
Many of Janssen's buildings also boast museum-quality wrought-iron by noted Philadelphia artisan Samuel Yellin.
Janssen collaborated with Yellin for 25 years, resulting in gracious iron details in his most important projects.