Twenty-two of his designs are listed on the National Register of Historic Places, including the Oakland City Hall in Oakland, California and the Soldiers and Sailors Memorial Hall and Museum and University Club in Pittsburgh, Pennsylvania.

Hornbostel first earned distinction for his work with the Board of Estimate and Apportionment in New York City, assisting engineers in the design of bridges.

Between 1903 and 1917, he was responsible for the architecture of the Queensborough, Williamsburg, Manhattan, Pelham Park, and Hell Gate bridges—spans for both automobiles and trains.

His masterpiece, the Penn Central Hell Gate viaduct (1917), is considered by many to be one of the most beautiful railway bridges in the world.

Their Beaux-Arts scheme created an ordered, axial sequence of buildings despite the hilly topography.