Commissions in a period of rapid economic expansion and prosperity were plentiful and Fenoglio became extremely prolific, establishing his studio at 60, Via XX Settembre, where he designed some of the major Italian examples of Art Nouveau.

Fenoglio's work became increasingly recognizable by his strategic use of pastel colors, the ornament that alternates between floral subjects and circular geometric elements, and from the wide use of cement frameworks combined with the sometimes-daring decorative elegance of iron and glass.

[4] Fenoglio was also one of the organizers of the 1902 and 1911 International Expositions in Turin, but he was also active in the publishing field, numbering among the founders and serving as one of the most important contributors to the magazine L'architettura italiana moderna.

Fenoglio became vice-president of the well-known Impresa Porcheddu, of the Società Anonima Cementi del Monferrato, and a partner in the Accomandita Ceirano & Company, and managing director of the emerging Banca Commerciale Italiana.

[5] Among Fenoglio's best known works are the Villino Raby (1901);[6] the famous Villa Scott (1902),[7] a triumph of loggias, turrets, bay windows and oriels; and, above all, his own home, the Casa Fenoglio-Lafleur (1902);[8] considered by many to be "the most significant example of liberty style in Italy.

Among the best known commissions of Fenoglio's industrial architecture are the Conceria Fiorio (1900), the Stabilimento Boero (1905), the Fonderie Ballada (1906), the car factory of Officine Diatto (1907) and the large building of the first Italian brewery Bosio & Caratsch, with the attached manor house (1907).