Pier Giacomo Castiglioni was born in Milan, Italy in 1913. He graduated from the department of architecture at Milan Polytechnic in 1937.
He co-wrote a libel on the Milan Town Planning Scheme: "Fifty Significant Squares in the Milan of the Future" with the architect Carlo Pagani in 1938.
In 1938 Pier Giacomo Castiglioni and his elder brother, Livio, founded the "Architecture, Urban Buildings and Research into Industrial Design" practice in Milan, which the youngest brother, Achille, joined in 1944. All three Castiglioni brothers were interested in both technology and art. In 1952 Livio left the joint practice.
Until his untimely death in 1968, Pier Giacomo collaborated with Achille on numerous designer objects.
In 1957 the exhibition "Colors and Forms in Today's Home" was mounted in Como, where Achille and Pier Giacomo Castiglioni annoyed specialists with their vision of a modern lifestyle because what they showed was a colorful jumble of styles that incorporated old and new furnishings instead of uniformly styled interiors.
The two brothers were particularly successful with the lighting they created for Arredoluce, Flos, and Artemide. They playfully explored new possibilities for form, linking technical innovation and minimalist economy of means to produce highly functional objects which were just as aesthetically satisfying as they were practical. They designed the lamp "Arco" (1962; for Flos), which links the qualities of floor and hanging lamps.
The Castiglioni brothers not only exerted a strong influence on the younger generation of Italian designers. From 1946 until his death Pier Giacomo Castiglioni was lecturer and assistant in architectural composition, professor in drawing and relief work at the Milan faculty of architecture.
The New Yorker Museum of Modern Art collection owns many works by Achille and Pier Giacomo Castiglioni.
Pier Giacomo Castiglioni received numerous prizes and awards, including the Compasso d'Oro five times.
He exhibited at every Milan Triennale from 1940 on, winning many awards. For the latter, based on the theme "Leisure time, living time", Pier Giacomo Castiglioni was a member of the Executive Committee.