Pier Giacomo Gastiglioni graduated in architecture from Politechno di Milano in 1937 and founded a design studio together with his brothers Achille and Livio and Luigi Caccia Dominioni. Pier Giacomo later became a lecturer in architectural composition as well as professor in drawing and relief work at the faculty of architecture at Politechno di Milano. During his career he was a member of the board at the Association of Architects and at the Milan City Council Building Commission.
Castiglioni worked on several committees for exhibitions and fairs such as the XIII Triennale in 1964 and exhibited at every Milan Triennale from 1940 onwards. As a exhibitions curator he worked for both national as well as international institutes and leading companies. In 1953 Livio Castiglioni left the studio while Achille and Pier Giacomo continued their work, and in 1957 they designed the Mezzadro seat and the Sella stool both produced by Zanotta.
Three years later the brothers created the San Luca armchair for the manufacturer Gavina and they started a long-lasting collaboration with Flos when they designed the Viscontea suspension light with sprayed-on resin lampshade. Pier Giacomo Castiglioni received the design award Compasso D’Oro five times before his death in 1968 and was honored the award posthumously in 1979.