“Williamsburger” is the name of an inhabitant of Williamsburg, in the borough of Brooklyn. The name “Williamsburger” has been cited in print since at least 1860.
   
 
Wikipedia: Williamsburg, Brooklyn
Williamsburg is a lower middle class neighborhood in the New York City borough of Brooklyn, bordering Greenpoint to the north, Bedford–Stuyvesant to the south, Bushwick and Ridgewood, Queens to the east and the East River to the west. The neighborhood is part of Brooklyn Community Board 1. The neighborhood is served by the New York Police Department (NYPD)‘s 90th Precinct. In the City Council the western and southern part of the neighborhood is represented by the 33rd District; and the eastern part of the neighborhood is represented by the 34th District.
 
Williamsburg is an influential hub for indie rock, hipster culture, and the local art community. Many ethnic groups also have enclaves within Williamsburg, including Italians, Jews, Puerto Ricans, and Dominicans. The neighborhood is being redefined by a growing population and the rapid development of housing and retail space particularly catered to a wealthy population that has been historically foreign to the neighborhood.
 
24 March 1860, Frank Leslie’s Illustrated Newspaper, pg. 256, col. 3:
EDITORIAL GLANCES AT MEN AND THINGS.
The Williamsburg and Brooklyn papers are very bitter upon George Law, on account of his conduct in the ferry-boats. The Williamsburgers have none to blame but themselves; ...
 
3 May 1890, New York (NY) Times, “Preservers of the Peace: Brooklyn Policemen Protect Themselves From Each Other”:
A pitched battle with clubs between two Brooklyn policemen furnished no end of entertainment yesterday morning to wondering throngs of Williamsburgers.
 
24 April 1918, New York (NY) Times, “From North to East”:
To the Williamsburgers and Greenpolnters; also, the new situation of the Sound boat.piers, ...
 
Google Books
The DeLongs of New York and Brooklyn;
A Huguenot Family Portrait
By Thomas A. DeLong
Southport, CT: Sasco Associates
1972
Pg. 78:
Jeff, a native Williamsburger born August 31, 1869. became a woolen and textile mill representative with an office at 79 Fifth Avenue, New York (1912).
 
Google Books
From Suburb to Shtetl:
The Jews of Boro Park
By Egon Mayer
Philadelphia, PA: Temple University Press
1979
Pg. 36:
These several factors jointly contributed to making Boro Park the choice of Williamsburgers and Crown Heightsers, and thus a center of Jewish Orthodoxy in the 1960s.
 
New York (NY) Times
Has Billburg Lost Its Cool?
By DENNY LEE
Published: July 27, 2003
(...)
Plenty of hipsters are still ardent Williamsburgers and would never live anywhere else.
 
The Brooklyn Paper
February 16, 2012
W’burg waterfront gets a supermarket
BY AARON SHORT
(...)
Community Board 1 member Ward Dennis said the community could use another grocery store because it is “still woefully under served” for food shopping — and he expects it will be popular with Williamsburgers living along the waterfront.