The area known today as Florida was ruled by four nations: France, Spain, Great Britain, the and United States between the years 1559 and 1819. In 1821, the US bought Florida from Spain for a total of five million dollars. On January 25, 1834, Hillsborough County was created from Monroe and Alachua Counties during the territorial period of the US (1822-1845). The area covered by Hillsborough County in 1834 was much bigger than the current area, and it included eight other present-day counties, including Pinellas, Pasco, Polk, Sarasota, Hardee, Manatee, Charlotte, and DeSoto.
In 1846, the Commissioners approved plans to build a new courthouse in Hillsborough County because the first courthouse was burned down during the second Seminole war. A two-story courthouse was built, and, in 1892, a new red brick courthouse was built in Downtown Tampa. The current administrative building for the court system known as the George E. Edgecomb was opened in 2004. The current county government administrative building, which opened in 1994, was renamed the Fredrick B, Karl County Center after the county administrator who played a key role in buying that building.

Hillsborough County gets its name from Wills Hill, who was the Earl of Hillsborough. Hill served as the colonies’ British Secretary of State in the years 1768 to 1772. Wills Hill was the Secretary of State for the Southern Department from 1779 to 1782. In 1789, Wills Hill was made the Marquess of Downshire in the Irish peerage. There are also other counties and towns in the United States that are also named after Wills Hill. They include Hillsborough County in New Hampshire, Hillsborough Township in New Jersey, the town of Hillsborough in New Hampshire, and the town of Hillsborough, North Carolina.