The region includes the following modern counties within its bounds: Andrew (435 square miles ), Atchison (545 square miles,), Buchanan (410 square miles), Holt (462 square miles), Nodaway (877 square miles), and Platte (420 square miles). It also includes the northwest suburbs of Kansas City, a small area of Kansas City proper, the cities of St. Joseph, Mound City and Maryville, Missouri, as well as Kansas City International Airport and almost all of Missouri's portion of Interstate 29, save the small portion which runs concurrently with Interstate 35 in Clay County.
Native American people living in unorganized portions of the Missouri Territory had been promised that they would keep their land permanently. However, White settlers quickly broke the agreement, including most notoriously Joseph Robidoux, and demanded incorporation of settled territory into the recently formed state of Missouri that was located east of the Missouri River to the latitude of the state's northern border.
An agreement was reached with Chiefs Mahaska and No Heart of the Ioway tribe and leaders of the combined Sac and Fox tribes in a ceremony that was presided by William Clark (of Lewis and Clark Expedition fame) at Fort Leavenworth, Kansas.
The tribes were paid $7,500 for their land. The U.S. government was to "build five comfortable houses for each tribe, break up 200 acres of land, fence 200 acres of land, furnish a farmer, blacksmith, teacher, interpreter, provide agricultural implements, furnish livestock" and a host of other small items. The tribes agreed to move to reservations west of the Missouri River in what was to become Kansas and Nebraska, today known as the Ioway Reservation and the Sac and Fox Reservation.
The western border of Missouri was established at the mouth of the Kaw River in Kansas City . The purchase extended the Missouri border in the northwest to 95 degrees 46 minutes West longitude).
On 28 March 1837, President Martin Van Buren proclaimed the Platte Purchase part of the state of Missouri, making what was the largest state in the union at that time even larger.
Current maps show the eastern border of Platte County and all counties north further east than the border between Missouri and Kansas south of the river, which no longer conforms to the mouth of the Kaw River.