Breckenridge County was created by the first territorial legislature in 1855 and named for John C. Breckenridge, who was the next year elected vice-president of the United States. When first created it was attached to Madison county for all civil and judicial purposes, but by the act of Feb. 17, 1857, the county was fully organized "with all the rights, powers and privileges of other organized counties of the territory; and the county seat of Breckenridge is hereby temporarily located at Agnes city," etc. The act of Feb. 27, 1860, provided for the location of a permanent county seat by vote of the electors of the county.

As originally established, the county was 24 miles square, lying immediately south of Richardson (now Wabaunsee) county, but by the act of Jan. 31, 1861, the southern boundary was moved southward to the line between townships 21 and 22 south. On Feb. 5, 1862, the governor approved an act changing the name of Breckenridge to Lyon county. (See Lyon County.)