area moved from Loudoun to Fairfax, 1798
When Fairfax County was created in 1742, the first courthouse was located near modern Tyson's Corner . The site of executions for criminals was nearby, according to local legend - and that's why "Courthouse Road" and "Gallows Road" exist in that area. Five years after Fairfax County was established, in 1749, the colonial General Assembly chartered the town of Alexandria and (in Prince William County) the town of Dumfries.
Both towns were sponsored by Scottish merchants, and shared the same street names - King, Duke, Prince, etc. The Scottish merchants planned to establish commercial operations to buy tobacco in the colony of Virginia, and to pay for it immediately (primarily by issuing credit so the farmers could purchase items immediately from the merchants' stores in Alexandria and Dumfries). This was a new business practice intended to compete with the traditional colonial pattern. Big planters like the Lees and Carters were forced to ship their tobacco across the Atlantic Ocean, have English merchants sell the tobacco (often at what the Virginian's considered a too-low bargain price), and finally wait for those merchants to issue credit for the sale price. .
Alexandria quickly boomed as a seaport with direct connections to Europe. Shortly after its formation, the population of Alexandria had more political clout than the population in the western portion of Fairfax County. In 1752, when the 1742 courthouse needed to be replaced, the Alexandria merchants convinced county (and colonial officials in Williamsburg) to move the county seat to Alexandria. Naturally, residents in western Fairfax were disgruntled by the shift, and they got a new Loudoun County created in 1757.
The eastern border of Loudoun County was defined by the headwaters of Difficult Run, then a line south to Bull Run (where Cub Run meets Bull Run, near modern-day Centreville Road). The courthouse for Loudoun County was located at Leesburg, too far away to suit the residents between Difficult Run and Sugarland Run.
In 1798, a portion of eastern Loudoun County was transferred back to Fairfax County. Presumably as part of the plan, the courthouse was also moved in 1800 to the new center of Fairfax. After all, transfer of a chunk of Fairfax County to the new District of Columbia would have left the county's courthouse in a jurisdiction outside the state of Virginia...
When the portion of the District of Columbia was "retroceded" or rfeturned to the state of Virginia, a new "Alexandria County" was created. It had its own courthouse in the city of Alexandria. To reduce confusion between Alexandria County and Alexandria City, the county changed its name to Arlington in 1920.
Read The Debates over the Retrocession of the District of Columbia, 1801–2004
A similar challenge emerged in 1961, when the Town of Fairfax became an independent city. To prevent the courthouses from being located outside the jurisdiction of the county, there is a "hole in the doughnut" of Fairfax County land located inside the boundaries of the city. The historic and the current county courthouses, and some property next to the courthouses, is still outside the city boundaries.
So Fairfax has had three courthouses at "Springfields" near Tysons Corner, in Alexandria, and in what is now a patch of county land inside the City of Fairfax. When other government operations became too cramped in offices shared with the judicial center, Fairfax County built a new "Government Center" west of the courthouse and moved most county staff to that location.
(Prince William County has a similar history, with 5 separate courthouses as population centers have shifted. The Prince William County courthouse facilities are located within the boundaries of the City of Manassas, which became independent in 1975, but most county offices are located at McCoart Administrative Center several miles to the east of the city. Where are most of Loudoun's county staff locate, and does it matter that Leesburg is still a town rather than an independent city?)