The Connecticut Colony (Hartford) and the New Haven Colony were two separate colonies until 1662, when a charter from King Charles II united them. According to Guide to the History and Historic Sites of Connecticut by Florence S. M. Crofut (New Haven: Yale University Press, 1937), Hartford was the only capital of the new unit until 1701. From 1701 to 1874, there were two capitals in CT, New Haven and Hartford. (Pp. 577, 636) Each city had the responsibility for different parts of the government, though the General Assembly always met in Hartford. As Crofut states (p. 637):
“The ‘single capital contest’ began in 1866, when the General Assembly appointed commissioners to investigate the state houses in New Haven and in Hartford. New Haven offered a site, if the State would appropriate $500,000 for the building. The more liberal offer of Hartford was accepted … and, in 1873, it was voted by the General Assembly that there should be only one capital and that it should be located at Hartford. The vote was ratified by the people as a Constitutional Amendment to take effect in 1875.”
It may be said, then, that Hartford has always been the capital of Connecticut, both as a colony and a state. However, New Haven was the capital concurrently with Hartford, from 1701-1874.