Which county government building was Wright's largest public commission?

Frank Lloyd Wright's largest public commission was the Marin County Civic Center.

The sprawling government complex is in San Rafael, California, and was commissioned by Marin County in 1957. It was intended to bring numerous county departments together on one campus, making it far more than a single courthouse or office block. Wright's plan incorporated landscape, civic buildings, circulation routes, and public spaces as one unified composition.

The principal structures include the long Administration Building, the Hall of Justice, a rotunda, and later cultural and public-use facilities. Their shallow arches, gold-colored details, blue roof surfaces, and bridges across the site give the complex a distinctive appearance. The design also drew on Wright's Broadacre City ideas about decentralized, landscape-integrated communities.

Wright died in 1959, before construction was finished, so protégés including Aaron Green and Taliesin-associated architects helped carry the plans forward. The complex was built in stages from 1960 into the 1970s. The Guggenheim Museum is more famous internationally, but the Marin project was Wright's largest public project and his major government commission.