The county was organized in 1841 as Niangua County, named after the river which bisects it south to north.  In 1844 it was renamed Dallas County, after U.S. Vice President George M. Dallas.

Buffalo, the county seat, was founded in 1841 on Buffalo Head Prairie.  The prairie, and subsequently the town, was named for a buffalo skull landmark erected by the first settler, Mark Reynolds, in 1833 near the present junction of U.S. Highway 65 and Missouri Highway 32.

A pro-Union county during the Civil War, Dallas County suffered many guerilla raids.  In October, 1863, the county courthouse in Buffalo was burned by Confederate troops under the command of General Joseph O. Shelby.   In July, 1864, Buffalo was again Struck by Confederate Raiders, who burned the Methodist Church, which was being used as the courthouse.

After the Civil War, a railroad was expected to come through Buffalo.  The county issued $250,000 in bonds in 1868-1871 and contracted with the Laclede and Fort Scott Railroad for construction of the rail bed and bridges.  Remnants of these are still visible from Highway 32 along the east-west route.

But, no track was ever laid, and the county refused to pay bondholders for the railroad that was never completed.

A compromise was reached shortly after the turn of the century, but not until the dispute had been carried to the U.S. Supreme Court and county officials either eluded or were arrested by U.S. Marshals.  The county debt for the railroad had reached more than $3 million by 1918.  Ultimately, a compromise figure of $300,000 was agreed upon and finally paid off in 1940.