As a junior partner he was lead architect in Reed & Stem's partnership with Warren and Wetmore to design Grand Central Terminal during its construction, starting in 1903.
Following the death of Charles Reed in 1911 he became a named partner of Stem & Fellheimer which designed Union Station (Utica, New York) in 1913.
The firm became Fellheimer & Long with Allen H. Stem Associated Architects in 1914 and designed the Morris Park (IRT Dyre Avenue Line) in the Bronx.
The firm became Fellheimer, Wagner & Vollmer which designed the Farragut Houses project in Brooklyn starting in 1942.
In 1951, the firm designed a new Montclair, New Jersey branch store with Roland Wank for Newark-based Hahne & Company.