County	Date Formed	Parent County	County Seat
Beaverhead	1865	Original County	Dillon
Big Horn	1913	Rosebud & Yellowstone	Hardin
Blaine	1912	Choteau	Chinook
Broadwater	1895	Jefferson & Meagher	Townsend
Carbon	1895	Park & Yellowstone	Redlodge
Carter	1917	Fallon	Ekalaka
Cascade	1887	Chouteau & Meagher	Great Falls
Chouteau	1865	Original County	Fort Benton
Custer	1877	Big Horn	Miles City
Daniels	1920	Sheridan & Valley	Scobey
Dawson	1869	An original county	Glendive
Deer Lodge	1865	Original County	Anaconda
Fallon	1913	Custer	Baker
Fergus	1886	Original County	Lewistown
Flathead	1893	Missoula	Kalispell
Gallatin	1865	Original County	Bozeman
Garfield	1919	Dawson	Jordan
Glacier	1919	Teton	Cutbank
Golden Valley	1920	Musselshell & Sweetgrass	Ryegate
Granite	1893	Deer Lodge & Missoula	Philipsburg
Hill	1912	Chouteau	Havre
Jefferson	1865	Original County	Boulder
Judith Basin	1920	Cascade & Fergus	Stanford
Lake	1923	Flathead & Missoula	Polson
Lewis and Clark	1865	Original County	Helena
Liberty	1920	Chouteau & Hill	Chester
Lincoln	1909	Flathead	Libby
McCone	1919	Dawson & Richland	Circle
Madison	1865	Original County	Virginia City
Meagher	1867	Chouteau & Gallatin	White Sulfur Springs
Mineral	1914	Missoula	Superior
Missoula	1865	Original County	Missoula
Musselshell	1911	Fergus, Meagher & Yellowstone	Roundup
Park	1887	Gallatin	Livingston
Petroleum	1925	Fergus	Winnett
Phillips	1915	Blaine & Valley	Malta
Pondera	1919	Choteau & Teton	Conrad
Powder River	1919	Custer	Broadus
Powell	1901	Deer Lodge	Deer Lodge
Prairie	1915	Dawson & Fallon	Terry
Ravalli	1893	Missoula	Hamilton
Richland	1914	Dawson	Sidney
Roosevelt	1919	Sheridan	Wolf Point
Rosebud	1901	Custer	Forsyth
Sanders	1905	Missoula	Thompson Falls
Sheridan	1913	Valley	Plentywood
Silverbow	1881	Deer Lodge	Butte
Stillwater	1913	Carbon, Sweetgrass & Yellowstone	Columbus
Sweet Grass	1895	Meagher, Park & Yellowstone	Big Timber
Teton	1893	Chouteau	Choteau
Toole	1914	Hill & Teton	Shelby
Treasure	1919	Rosebud	Hysham
Valley	1893	Dawson	Glasgow
Wheatland	1917	Meagher & Sweetgrass	Harlowton
Wibaux	1914	Dawson, Fallon & Richland	Wibaux
Yellowstone	1883	Custer	Billings