The Italian designer and architect Pier Giacomo Castiglioni is the second of the three Castiglioni brothers. Like his brothers, Pier Giacomo Castiglioni studied architecture at Milan Polytechnic. After their graduation, Pier Giacomo Castiglioni and his elder brother, Livio, founded a practice in Milan in 1936. At the beginning they worked with Luigi Caccia Dominioni. The youngest Castiglioni brother, Achille, joined the practice in 1944. All three Castiglioni brothers were interested in both technology and art. They worked together until 1955, when Livio left the practice and the studio took the name "A.& PGCastiglioni"

In 1957 the exhibition "Colori e forme nella casa d'oggi" (Colors and Forms in Today's Home) was mounted at the Villa Olmo 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. Achille and Pier Giacomo Castiglioni showed many of their designs, including for the first time their "readymades". The two brothers were particularly successful with the lighting they designed 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. In 1955 the Castiglionis designed "Luminator" for Arredoluce; their "Taraxacum" hanging lamp dates from 1960 and the hanging lamp "Splugen Brau" for Flos was launched in 1961. Another Castiglioni design for Flos was the 1962 die lamp "Arco", 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. Pier Giacomo Castiglioni taught design at Milan Polytechnic from 1946 until his death.