When was the last time the boundaries of a U.S. county (or parish, borough, etc.) were changed? Are county boundaries normally set up before a territory becomes a state or afterward? What are some of the reasons for changing county boundaries once they are established?
They tried to make a minor change in the county boundary around here, but it was such a long and involved process (you needed approval from both the county and town legislatures in both counties) that they gave up. It possible, but the hoops are big, since the change usually affects the tax base.
In this case, it was because the Rotterdam Eagles – a social club – bought land for a new clubhouse that was a few dozen feet over the border in the town of Guilderland. They were a Rotterdam organization and wanted to remain in the township, but since the land crossed the county line between Schenectady and Albany Counties, everyone had to agree. There was supposed to be some land swap to equalize, but it grew too complicated and the Eagles landed elsewhere in Rotterdam.
I can’t say for all states, but for one example, in New York the counties were established before the founding of the US. Albany County was founded in 1683 (as were all of New York’s original 12 counties). The twelve counties were broken up over time; Schenectady County was separated from Albany County in 1809. The two newest counties in New York are Wayne and Yates, both founded in 1823.
Nassau County was split off Queens County in 1899, taking the large chunk of Queens County which lied outside the newly formed New York City borough of Queens.
Peach Co., GA, was founded in 1924 from parts of Macon and Houston Counties.
Loving Co., TX: made from Tom Green Co. in 1931
Here’s the Wikipedia list of proposed county changes, but in that list, Milton County was merged with Fulton County in 1932 to save money during the Great Depression.
There are few other splits and merges in the list as well.
There has been talk in Nebraska for several years of consolidating many of the more rural counties in the western part of the state. A bill to start the process died in the state legislature this spring. A lot of the opposition came from the fact that some of the existing counties are as large as 1,000 square miles, and providing “local” services to even larger areas would be impossible.
In Missouri, the City of St. Louis was carved out of St. Louis County in 1876. There were several other counties formed out of parts of one or more counties as late as 1859. Those counties seem to be along major railroad lines, and my WAG is that as population increased along the lines, it made sense to redraw the county lines to take in the new population patterns.
Broomfield County Colorado was carved out of parts of four other counties in 2001. The city of Broomfield, in Boulder County, had grown to cross over county lines, and residents were tired of dealing with different governments for different neighborhoods.
Every time a state border changes in the US, and they do frequently, a county border is changing. Changes occur due to more accurate survey techniques, and boundaries based on changing natural formations. 
You were probably more interested in intentional changes, but I’m just in the mood to give an answer like that. Sorry.
You might find "How the States Got Their Shapes " (ISBN-13: 9780061431395) interesting. Many state-border changes would also have affected bordering counties. Example; One kink in the Massachusetts border was to cede an area to NY; MA couldn’t police it because of terrain difficulties, and bad guys were holing up there, IIRC.
Out here in Montana, an act was passed in 1915 that made it absurdly easy to make a new county and, before it was repealed in 1925, they made 20 new counties out east. This was during the height of settlement on the plains and most of these new counties have been losing population ever since they were founded and a couple of them have less than 1,000 residents these days (Petroleum County only had 440 people as of the last census).
The main motivator for changing these county boundaries is that each new county got its own state senator, so it was basically a way to pack the state senate.
This page might help. It’s a list of substantial changes (affecting at least 200 people or one square mile) to counties and county equivalents from 1970 to today:
This is similar to the history of county formation in Kentucky. Before the State Constitution reined things in in 1891, county subdivision was rampant. It’d be interesting to find out whether it’s actually possible to form a new county in Kentucky under the current restrictions on size, population, and boundaries.
County boundaries are set by the State Legislature, so they could change them at any time.
Here in Minnesota, a fairly large city, St. Cloud, happens to be divided between 3 counties. There was a proposal last year to create a new county that included all of the town & nearby area, and then to combine the remainder of those 3 counties. It hasn’t passed, yet. But it will likely be considered more next Legislative session.
So changes are still happening.
Denver County took land from Adams Country to make the new airport so we’re up to early 1990’s
I don’t think so. If the original survey is accepted then any changes must be approved by both states and Congress, even if the original survey was incorrect. State borders simply do not change as easily as you seem to think.
Not true. As Freido pointed out, Nassau County was created in 1899. The newest NY county is Bronx County; it was carved out of New York County in 1914.
The last one that involved me was in 1997, when the Montgomery and Prince George’s Counties boundary was moved to place Takoma Park wholly inside Montgomery County.
This was in Maryland, BTW. Around the same time, a Burtonsville neighborhood was moved from Montgomery to PG county so a restauranteur wouldn’t be faced with back-taxes from Montgomery County. That particular frontier is a work-in-progress.
An odd one.
The western county boundary for Summit County and eastern Eagle County Colorado is defined by the continental divide at one point. Climax mine (molybdenum) ended up actually moving the continental divide. Similar to a river changing it’s course I guess, but this is a little different.
My memory (admittedly faulty) says that one state reorganized all their counties and redrew almost all the county lines, in the process reducing their number by about one third. My memory further says that the state in question was Georgia and the time they did this was the early 20th century, although it may have been the 19th. It’s also possible that it was some other state.
Unfortunately, I haven’t been able to find any cites on the Infobahn to support this memory. Anyone able to help my memory or should I just tell it that it’s imagining things?