Sumner County was formed in 1867:
"Another act provided for the division into counties of all the unorganized part of the state east of range line 26 west; the counties to be organized when they should have the requisite population. The counties created by this act were: Montgomery, Howard, Cowley, McPherson,Sedgwick, Sumner, Jewell, Mitchell, Lincoln, Ellsworth, Rice, Reno, Harper, Smith, Osborne, Russell, Barton, Stafford, Pratt, Barbour, Phillips, Rooks, Ellis, Rush, Pawnee, Kiowa, Comanche, Norton, Graham, Trego, Ness, Hageman, Ford, and Clark. By the same act Seward disappeared, and the boundaries of Greenwood, Butler and Marion were changed. Butler took its final form. The south line ofGreenwood was pushed nine miles south, to the north line of township 30. Marion was confined to one tier of townships on the south, taken from Butler (id. pp. 51-57)."
From "Transactions of the Kansas State Historical Society, 1903-1904"; edited by Geo. W. Martin, Secretary. Vol. VIII. Topeka: Geo. A. Clark, state printer, 1904.
Prior to 1867, the land that is now Sumner County was in Butler County. For information prior to that, also search in Butler County records.