City of Hartford data is from the CT.gov website in the Secretary of the State section. The cells shaded pink, essentially 1880 and earlier, had figures that were in slight conflict with the numbers on Wikipedia, though the approximate magnitudes were the same.
County data since 1900 is from the census.gov website, which matched the figures available on Wikipedia. Data from before 1900 has not been confirmed on either a State or Federal site, and comes directly from Wikipedia.
State data is from the CT.gov website in the Secretary of State section. The numbers were confirmed with the census.gov between 1900 and 1990, and also matched the Wikipedia numbers for the full date range.
CT Secretary of the State
Population by Town from 1756 – 1820
Population by Town from 1830 – 1890
Population by Town from 1900 – 1960
Population by Town from 1970 – 2010
County and State Population from 1900 to 1990
City of Hartford Quick Facts Page
City of Hartford
State of Connecticut