If you are looking at demographic or economic trends over time by city or county, the U.S. Census can be a great source of information. Unfortunately, Census data prior to the year 2000 can be challenging to easily retrieve.
I've found that the County and City Data Book or Statistical Abstract of the United States to some of the easier sources to use when trying to find older Census data. It's a bit of a time consuming process, but the directions below are pretty easy to follow.
Note: The directions below are for the County and City Data Book, but the same process will work for the Statistical Abstract of the United States.
Using the County & City Data Book via Hathi Trust, choose the volume/year that you need.
Use the Keyword search box to search for your City or County
Click the links to specific pages with your location name. You may need to look at multiple pages within the volume to find what you are looking for.