Summit County, Ohio is where LeBron James grew up and played basketball, including his noteworthy high school career.
Animal House was filmed here.
Salt Lake County, Utah: final settling place of the Mormon pioneers, in 1847, after being chased across the country.
We made the national news when:
Our local nutbags protested a new English textbook on the grounds that it promoted witchcraft
Students at the middle school got involved in a sexting scandal
A Spanish teacher at the high school slept with one of her students
Several times when local high school baseball players got drafted by the majors
We’re just a hotbed of news around here.
Where I am now:
Wreck of the Ten Sails- pretty much as the names sosunds. In one night, ten ships in a convoy bound from Jamaica to England hit the reef and sank off the East End of Grand Cayman in February 1794. Amazingly for the time only eight sailors died.
Where I grew up:
The local Eastman Chemical Company provided the management and expertise to run the Y-12 complex during the Manhattan Project. It is certainly the most important and newsworthy item out of my old home county but was never discussed at the time and rarely afterwards. Wartime secrecy?
The murder of the three RCMP officers in Moncton New Brunswick in early June. He just plead guilty today of 3 counts of 1st degree murder and 2 counts of attempted murder.
West Chester, OHIO - Home of a current desperate shortage of spray tan. 
I’d tell you, but you’d think it was total bunk.
Bexar County, TX - Remember the Alamo!
Where I lived the longest - Conroe, Texas (Montgomery County) - I would sadly say the Clarence Brandleycase.
Where I live now? Most recently Casey Kasem’s final days put little Port Orchard, WA (South Kitsap County) on the map.
I grew up in the same small town as Megan Meier (Suicide of Megan Meier - Wikipedia), although I’d moved away years before her suicide. That was probably the most noteworthy thing from that county in recent memory. Going back a bit, Lewis and Clark stopped in St. Charles for supplies at the start of their expedition. I’m sure lots of other noteworthy things have happened in the county in the past 200 years, but the Megan Meier thing was what popped into my mind.
Washoe County Nevada (Reno)….
Reno Air Races crash in 2011.
Flood on New Year’s Day 1997 which I found out about because there was a huge color picture of it in the newspaper in Auckland, New Zealand.
The US invasion in 1989 was pretty big news…
Sacramento, CA
It’s hard to say, but the first thing I thought of was the assassination attempt on President Gerald Ford, by Manson family member Lynette “Squeaky” Fromme.
Jaycee Dugard was found in Contra Costa County eighteen years after she was kidnapped.
The Port Chicago disaster and mutiny happened in Contra Costa County.
George Washington was here when he received notification that he had been selected as our first US President. He also lived here, but that didn’t make the papers in and of itself. . .
ETA: I should note that the County has since been divided, but it was all one at the time. . .
OP, do you mean today, or ever?
I’ma go with the opening of Disneyland, on the future kaylasmom’s second birthday.
Cambria County, PA, here. We seem to have a problem with our rivers. You may remember the floods we had in 1977 and 1936. I’m sure you’ve heard of the one we had in 1889 that killed over 2200 people.
I grew up in Pierce County, which can lay claim to the Tacoma Narrows Bridge collapse. Everybody has seen that film.
The most newsworthy event in my town that happened to a friend of mine was the murder of Vaugh Henry, son of Dawn Breedon, by his father.