Duval County was created in 1822 from St. Johns County. It was named for William Pope DuVal, Governor of Florida Territory from 1822 to 1834. When Duval County was created, it covered a massive area, from the Suwannee River on the west to the Atlantic Ocean on the east, north of a line from the mouth of the Suwannee River to Jacksonville on the St. Johns River. Alachua and Nassau Clay County was created from part of Duval County in 1858. Part of St. Johns County south and east of the lower reaches of the St. Johns River was transferred to Duval County in the 1840s.        

On  October 1, 1968, the government of Duval County was consolidated with the government of the city of Jacksonville, although the Duval County cities of Atlantic Beach,Baldwin, Jacksonville Beach, and Neptune Beach are not included in the corporate limits of Jacksonville, and maintain their own municipal governments.

Wikipedia... http://en.wikipedia.org/wiki/Duval_County,_Florida