Jefferson County, one of the counties formed and organized by the first territorial legislature, is situated in the northeastern part of the state, the second county west from the Missouri river and the third south from the Nebraska line. It is bounded on the north by Atchison county, on the east by Leavenworth, on the south by the Kansas river, which separates it from Douglas county, and on the west by Shawnee and Jackson counties.

Jefferson is one of the older settled counties of the state and some of the most important events in the history of Kansas took place within its borders. The first visit of white men of which there is any record is the expedition of Prof. Say, which entered the county at the southwest corner of Delaware township and proceeded to the falls of the Delaware (then the Grasshopper) river, where camp was made on the night of Aug. 27, 1819. The next day they crossed the northern boundary. The first settlement was made by Daniel Morgan Boone, son of the famous Kentuckian, who was appointed "farmer for the Kansas Indians" by the government. He located in 1827 on the north side of the Kaw river in the extreme southern part of what is now Jefferson county, and started to teach the Kansas Indians the art of agriculture. His son, born on Aug. 22, 1828, was probably the first white child born in Kansas. Boone maintained the first agency for Indian lands in the state. Subsequently a settlement grew up, the ruins of which were found near the present village of Williamstown by settlers in 1854.

In 1851 a few Mormon families en route from Missouri to Salt Lake stopped in Jefferson county, about where Thompsonville is now located. They remained about two years and made some improvements. Three log cabins were built and about 15 acres of land cultivated. Three of the women in the company died of cholera. They were buried in the edge of the timber and tombstones put up with the names cut on them. One was a Mrs. Archer and one a Mrs. Platt. Finding they could not obtain the lands in Kansas they moved on.

Permanent settlements were made in 1854, a military and freight road having been opened that year between Fort Leavenworth and Fort Riley. The following are some of the settlers who came in that year: William F. and George M. Dyer, Henry Zen, Henry Chubb, William B. Wade, Sidney Stewart, Aaron Cook, R. P. Beeler, Jefferson Riddle, J. T. Wilson, John Kuykendall, John Scaggs, Thomas R. and Alexander Byne, Charles Hardt, Simeon and Isaac Hull, Charles Hedrick, John Hart, J. B. Ross, Robert Riddle, James Frazier, A. J. Whitney and T. J. and H. B. Jolley. The settlements were made along the government road and the Kansas river, and at the crossing of the Grasshopper. The lands had not yet been opened for sale, but the immigrants paid no attention to that fact. They staked off claims and began improvements, with the understanding that when these lands were offered for sale they could bid in their holdings at the appraised value. This was the famous "squatter's right" that caused so mach trouble in territorial days, and this condition obtained with nearly all the best lands in Jefferson county. An election was held that year, the polling place in Jefferson being at Ozawkie. The Missourians drove the free-state men from the polls. During the summer Congress established two mail routes across the county. One was along the old military road and the other was from Fort Leavenworth to the Big Blue by Grasshopper (now Valley) Falls. The first postoffice was Ozawkie, established on March 15, 1855, with George M. Dyer postmaster. Hickory Point was established soon afterward with Charles Hardt as postmaster and in Dec., 1855, Grasshopper Falls, with A. J. Whitney postmaster.

The first white child born after the permanent settlement was Ella Simmons, daughter of Mr. and Mrs. Alpha Simmons, June 19, 1855. The first marriage was between Alfred Corey and Miss Martha Hoovey at Ozawkie, Nov. 25, 1855.

In the election of 1855 Hickory Point was the polling place. Large numbers of pro-slavery men came into the territory the day before, camped near Hickory Point, laid off claims and the next day demanded the right to vote. On being refused a row ensued, and the election board finding it impossible to secure a fair election refused to serve. When the free-state voters came they found the polls in the hands of non-residents and went away without voting.

The first term of the district court in the county was held at Ozawkie the last week in March, 1856, with Samuel D. Lecompte as the presiding judge. At this time it was hardly safe to be abroad unarmed, as the border war was in progress and bands of armed men from other parts of the state and from Missouri frequented the settlements of Jefferson county. If a free-soiler was caught by a band of border ruffians he was apt to be killed. During the absence from Grasshopper Falls of the free-state defense organization the pro-slavery men visited the place, insulted the women and made various threats. In retaliation the free-state band under the leadership of Clark made a raid and killed a man by the name of Jackson, who was responsible. For the death of Jackson Grasshopper Falls was raided on Sept. 8, 1856, by a body of armed ruffians. The town was completely sacked and the store of William and R. H. Crosby was burned. Both sides were now in arms. Some South Carolinians who had been committing various depredations were discovered and captured on Slough creek, but were released on promising to leave the territory. The trouble culminated in the Battle of Hickory Point (q. v.).

The Jefferson county free-soilers took no part in the election of delegates for a constitutional convention in 1857. In August of that year, at the election for state officers, A. G. Patrick of Jefferson county was elected clerk of the supreme court. Two conventions were held at Grasshopper Falls in the latter part of August, one a mass and the other a delegate body, to discuss the contest for the control of the legislature. It was decided to put a full ticket in the field. (See Grasshopper Falls Convention.) The convention for Jefferson county was held at Ozawkie the same month.

The first county officers were appointed in 1855 as follows: Franklin Finch, probate judge; W. F. Dyer, treasurer; G. M. Dyer, sheriff; Marion Christison, register of deeds; William Sprague, assessor; Garret Cozine, surveyor; James A. Chapman, coroner; N. B. Hopewell, O. B. Tebbs and Henry Owens members of the county court, which was the same as the board of commissioners. Ozawkie was designated as county seat. At the meeting of the county court on Jan. 21, 1856, the county was divided into three townships, Slough, Ozawkie and Grasshopper Falls. The first road in the county was located in April and ran from Ozawkie east to Alexandria on Stranger Creek in Leavenworth county.

Early in 1858 the legislature authorized a county seat election. The contesting towns were: Oskaloosa, which received 177 votes; Grasshopper Falls, 173; Ozawkie, 94; Hickory Point, 50; and Fairfield, 10. Another election was held, in which Oskaloosa received 294 votes; Grasshopper Falls, 271; Ozawkie, 103; Hickory Point, 107; and Defiance, 3. A third election was held, between the two highest towns, in which Oskaloosa received the majority and was made the county seat. In 1864 the legislature again authorized a county seat election, in which Oskaloosa was again victorious. A court-house was built in 1867.

During the Civil war a regiment known as the Fourth Kansas militia was organized in Jefferson county, with S. S. Cooper as colonel. It comprised eleven companies, two from Grasshopper Falls, two from Jefferson township, two from Oskaloosa, one from Ozawkie, one from Rock Creek, one from Kaw, one from Kentucky and one from Sarcoxie. The object was the protection of the state from invasion. The county was also well represented in the Kansas regiments in the field.

The first schools were opened in 1859. J. H. Bennett was the first county superintendent and organized 9 districts that year. By 1865 there were 20 districts. After the war all the old districts were reorganized and 50 more added before 1870. New and well equipped school houses took the place of the log structures.

In the year 1860 the population of the county was 4,446, and the assessed valuation of property $925,003. The drought of 1860 and the war beginning the ensuing year retarded growth and development, but by 1870 the county had a population of 12,565, the assessed valuation of farm lands was $4,218,363, and the number of improved acres 91,004.

The area of Jefferson county is 568 square miles or 363,520 acres. It is divided into twelve townships, viz.: Delaware (formerly Grasshopper), Fairview, Jefferson, Kaw, Kentucky, Norton, Oskaloosa, Ozawkie, Rock Creek, Rural, Sarcoxie and Union. The general surface is undulating prairie with a few rough places. The bottom lands along the creek beds and the Kansas river comprise about 15 per cent. of the total acreage. The Kansas river flows east along the southern border except for about 6 miles of the southeast corner. The Delaware (formerly the Grasshopper) enters on the northern border about 4 miles from the west line and flowing south joins the Kansas at Perry. This stream is large enough to furnish power for mills. Big Slough creek is the main branch of the Delaware. Muddy creek crosses the southwest corner of the county.

Jefferson county is well supplied with railroads, the Leavenworth, Kansas & Western (Union Pacific) enters a few miles south of the northeast corner, crosses to Valley Falls and thence northeast into Atchison county; a branch of the Missouri Pacific enters in the east and crosses northwest through Valley Falls into Jackson county; a line of the Atchison, Topeka & Santa Fe enters from the southwest, crossing the county in a northeasterly direction, a branch diverging at Meriden and running east into Leavenworth county; and the Union Pacific crosses the extreme southern border, following the Kansas river. The total mileage of railroad is 121.44. The population of the county in 1910 was 15,826.