The U.S. Census Bureau has collected and summarized data every 10 years since 1790. While questions have changed, this resource provides a continual look at the U.S. population.
United States. Census (1790-1990)
Gov Docs, Lower Level C 3.240/7
Broken down by State, good if your city is smaller
The Statistical Abstract of the United States provides an authoritative and comprehensive summary of statistics on the social, political, and economic conditions of the United States. Detailed, monthly data is available from 2013 to present online. Historic data is available in print.
Statistical Abstract of the United States (1941-2012)
Gov Docs Lower Level, C 3.134