Marion County was organized on December 23, 1826 from Ralls County and was named for Revolutionary War folk hero Francis Marion. The county seat is in Palmyra.
MARION, a county in the E. N. E. part of Missouri, bordering on the Mississippi river, which separates it from Illinois, has  an area of 432 square miles. It is traversed by the North and South Fabius rivers, and by the North Two river, and also  drained by South Two river. The surface is somewhat diversified, and consists of undulating prairies, and forests which are  distributed along the larger streams. The prairies have a deep, rich soil, free from stones, and covered with a profusion of  flowers. Indian corn, wheat, oats, hay, hemp, cattle, pork, and butter are the staples. In 1850 this county produced  668,653 bushels of corn; 105,841 of wheat; 65,384 of oats; 4585 tons of hay, and 499 of hemp. It contained 23 churches,  2 newspaper offices, 1436 pupils attending public schools, and 277 attending other schools. [p.661] Bituminous coal and  saltpeter are found in the county, and limestone and freestone are abundant. A railroad is in progress of construction through the county from Hannibal to St. Joseph on the Missouri river. Capital, Palmyra. Population, 12,230, of whom 9398  were free, and 2832, slaves.
 Geographic Reference Library
 1854 Gazetteer of the United States
 United States Gazetteer

In 1817 Giles Thompson built the first cabin north of Salt River. . .The earliest settlers came from Kentucky, Virginia, and North Carolina. . .Marion County was named for General Francis Marion, and was organized as dependency of Ralls county, under act of Dec. 14, 1822. One trader named Smith had a store at the first hollow above the mouth of the bay. Marauding, drunken Indians killed him, and the tradition was that he left  a buried keg of money. Robert Masterson, who came here in 1818, gave the pointer for excavating, and much digging was done in the surrounding neighborhood in search of the treasure.