[6] Following the war, Castiglioni returned to Milan and joined the architectural design practice that his brothers Livio and Pier Giacomo had started with Luigi Caccia Dominioni in 1938.

[3] Much of their work was in exhibition design, but they also carried out a number of architectural projects, including the reconstruction in 1952–53 of the Palazzo della Permanente [it], which had been destroyed by bombing in 1943.

[3][6] Together, the brothers created a number of works that explored Marcel Duchamp's concept of the ready-made by incorporating and repurposing existing objects into new designs such as the "Sella [it]" and "Mezzadro" stools for Zanotta in 1957.

In 1959, they began working with Kartell, designing lighting and furniture, including a collection of tables and stools called "Rochetto".

[11][12]: 173 [13][14]: 79 [15]: 356  Also for Flos, in 1962 they designed both the "Toio" lamp,[16] assembled from "ready-made" surplus hardware, and the "Arco" lamp,[17] which consists of a long arched stainless-steel cantilevered support, an adjustable shade made of perforated spun aluminium, and a heavy marble base.

[20][21][22][23] In 1997, the Museum of Modern Art (MoMA) in New York staged a retrospective of his life and work titled: "Achille Castiglioni: Design!".

A total of c. 11,500 technical drawings and freehand sketches is complemented by 130 plastic models, boxes and drawers containing photographs, slides, glass plates and negatives, videocassettes, DVDs, audio cassettes, extracts from magazines, books, catalogues, and objects collected by Achille Castiglioni.