Green’s Dictionary of Slang

1. (US) a member of the upper-middle or mercantile class; thus attrib. in brownstone club, a private club; brownstone vote, the political stance of the upper-middle class.

1903		New Outlook 75 942: He went among his neighbors and appealed to them. The ‘brownstone’ vote came out.
1960		(ref. to 1870s) M. Berger Meyer Berger’s New York (2004) [ebook] In the Eighteen Seventies the term “brownstone” was more or less interchangeable with ‘blueblood’ in this city. A rich man was ‘a brownstone,’ flossy clubs had ‘brownstone membership’ and the ‘brownstone vote’ was the silkstocking vote.
1993		(ref. to 1870s) I.L. Allen City in Sl. (1995) 225: Brownstoner became an epithet in the 1870s for a well-to-do person, especially a striver of the merchant class. Brownstone also modified things, brownstone club for a private club or brownstone vote for the political inclinations of the parvenu class.

2. see Mr Brownstone under Mr n.