The first questions about income (beyond occupation and value of real estate) appear in the 1940 decennial census. (Encyclopedia of the Census, 260).
In 1940, income data is published for cities of 100,000 or more. Later, as the census relies more heavily on sampling for some of the data, and as census tracts and block groups are available as geographic levels, income data becomes available at the block-group level.
After 2000, income questions are no longer part of the decennial census and instead appear in the American Community Survey. See: the Cornell Library guide on the United States Census for additional information on searching the Census and the American Community Survey.
Access via the Cornell Premium Edition of Social Explorer or NGHIS.
Anderson, M. J. (2000). Encyclopedia of the U.S. census. Washington, DC: CQ Press. [Olin Library Reference, HA37.U55 C66x 2000 +]