According to infoplease.com, that first census, 1790, showed a population of 3,929,214.
For detailed census information for each ten-year mark from 1790 through 1960, see the "historical census...
The law establishing the 1790 US Census had allowed 9 months for the gathering of the census information. The Census was begun on 2 August 1790. A summary of the Census Results was transmitted to...