Deriving its name from George Montagu Dunk, 2nd Earl of Halifax (1716-1771) Halifax County was established by Order-in-Council on August 17, 1759. The boundaries of four other counties - Annapolis, Kings, Cumberland and Lunenburg - were specifically defined at that time, with Halifax County comprising all the part of peninsular Nova Scotia that was not within their limits.
Following the Seven Years War, Cape Breton Island was formally annexed to Nova Scotia. For a time it formed part of Halifax County.
The boundaries of Halifax County were modified in 1822. That part of St. Mary's Township (established in 1818) which had been in Halifax County was annexed to and included within Sydney County.
The boundaries of the County are also the boundaries of Halifax Regional Municipality.
The dividing line between the Districts of Halifax and Colchester was confirmed and established on May 3, 1828. In 1835, Halifax County was divided and the Counties of Colchester and Pictou County were created out of parts of what had hitherto been Halifax County. Eventually in 1880 the boundary between the Counties of Halifax and Colchester was fixed.