Leavenworth County, in the northeastern part of the state, is in the third tier of counties south of Nebraska and in the first west of the Missouri river. It was erected by the first territorial legislature with the following boundaries: "Beginning at a point on the southern boundary of Atchison county due north of a point 4 miles west of Dawson's crossing of the Fort Riley road on Stranger creek; thence due south to the main channel of the Kansas river; thence down said channel to where said channel crosses the channel of the Missouri river; thence up said channel of the Missouri, to the southeast corner of Atchison county; thence along the southern boundary of Atchison county to the place of beginning."

As thus defined Leavenworth embraced essentially the present county and that of Wyandotte, which was detached and erected as a separate county in 1859. It was named in honor of Gen. Henry Leavenworth, an officer in the United States army. At present the county is bounded on the north by Atchison county; on the east by Wyandotte county and the Missouri river, which separates it from Missouri; on the south by Wyandotte, Johnson and Douglas counties; and on the west by Douglas and Jefferson. It has an area of 455 square miles and is divided into the following townships: Alexandria, Delaware, Easton, Fairmount, High Prairie, Kickapoo, Reno, Sherman, Stranger and Tonganoxie.

In the eastern portion, along the Missouri river, the land is broken and hilly in places, but the western and southwestern portions are rolling prairie. The river and creek valleys average from a half mile to a mile and a half in width and comprise about one-fourth of the area. The uplands are somewhat sparsely wooded and artificial groves have been planted in many places. Timber belts along the streams contain all varieties of trees native to Kansas. The principal water courses are the Missouri and Kansas rivers. The largest tributary creeks are Little Stranger, Tonganoxie, Nine Mile, Salt, Wolf and Kaw. Leavenworth county is in the front rank in the production of Irish potatoes and sorghum. Winter wheat, corn, oats and hay are also extensively raised. There are over 300,000 fruit trees of bearing age in the county, and it also ranks high in live stock raising. Limestone and sandstone for building are abundant and are extensively quarried near Leavenworth. Gray limestone is found in the bluffs and blue limestone on the lower lands. Red and yellow ocher exist south of the city of Leavenworth; in the west and southwest cement rock of a good quality is plentiful; potter's clay is common in several localities; a great part of the county is underlaid with a bed of coal, which has for many years been mined at Leavenworth and at the state penitentiary at Lansing.

The first whites to visit the country now embraced within the bounds of Leavenworth county were the French traders and explorers who came up the Missouri river early in the eighteenth century. A Jesuit mission was established on the Missouri river, in what is now Leavenworth county, for the support of which "600 livres" were appropriated annually, and this was the first mission in what is now the State of Kansas. By 1757 the Missouri river route to the west was known to the French voyageurs and fleets of boats came down the river each spring laden with furs bought from the Indians at the headwaters.

In the spring of 1827, Col. Henry H. Leavenworth was directed to ascend the Missouri river and select a site for a permanent post. (See Fort Leavenworth.)

In 1833 a Methodist mission was established in what is now the northeastern part of Leavenworth county, and the following year the Catholic church established a manual labor school for the Indians in what is now Kickapoo township, but as they did not take kindly to work it was abandoned. Thus, outside of the fort, the missionaries were the first to locate permanently. In 1844 Maj. Robert Wilson established a trading post in Salt creek valley, but sold out in 1852. The first white settlers in Leavenworth county who had any legal right to locate within the territory were the farmers who came to cultivate the lands on the military reservation and the missionaries of the immigrant Indians. The pro-slavery residents of Platte and other Missouri counties, who favored the establishment of slavery in Kansas, knew of the rich and valuable land held by the Indians just across the line and only waited for its cession to come in and take claims. When the treaties were made they did not wait to learn the provisions for the sale of the lands, but rushed across the river and staked out claims. So great was this influx that by the close of June, 1854, there was scarcely any land that had not been claimed by settlers from across the border. The first land claimed after the passage of the Kansas-Nebraska bill, in what is now Leavenworth county, was that where the present city of Leavenworth (q. v.) stands. On June 10, 1854, the squatters who had taken claims near the fort in Salt creek valley held the first "squatter meeting" in the territory. Two squatters' associations—the Leavenworth County and the Kansas-Delaware association—were formed for the purpose of preventing non-residents from taking up land. At one of the meetings a disagreement arose between a man named McCrea, who had a claim near the fort, but not on the trust land, and a man named Clark. This led to a fight in which McCrea shot Clark so that he soon died. The pro-slavery men claimed that William Phillips handed McCrea the revolver, and he was ordered to leave the county. When he did not leave a vigilance committee, after warning him, took him to Weston, tarred and feathered him and rode him through the streets on a rail. This was really the first clash between the free-state and pro-slavery factions, but it did not deter settlers from coming into the county and making permanent homes. Subsequently several prominent free-state men of Leavenworth county were arrested upon various charges and ordered to leave the territory. On May 31, 1856, a meeting was called at Leavenworth at which a vigilance committee of pro-slavery men was appointed to notify some of the most active free-state advocates to leave the territory, but Gov. Shannon's proclamation ordering all committees organized for the purpose of driving settlers out of the territory to disband broke up the committee in Leavenworth county.

A band of regulators was formed in Leavenworth county, with Frederick Emery as captain, and a reign of terror was again commenced. Armed men patrolled the river and turned back boats which brought suspected free-state immigrants. The road from Leavenworth to Lawrence became known as "The Devil's Highway," and some of the pro-slavery men vowed that no free-state settler could travel on it between the two towns. Rev. Ephraim Nute, the Unitarian minister of Lawrence, John Wilder, a merchant of Lawrence, and a Mrs. Hoope were taken on the road and the men held as prisoners of war, the woman being allowed to go down the river. The "Regulators" under Emery killed William Phillips in the name of "law and order," entered the homes and stores of free-state men and turned them out of doors. Many of the citizens were placed on board vessels bound for St. Louis and other points down the river. When Gov. Geary arrived he soon put a stop to the actions of the regulators. From this time oppression and cruelty because of politics began to die down in the county, although hostilities were kept alive in the city of Leavenworth until violent means were used to bring about peace.

Leavenworth county was organized by the first session of the territorial legislature. When the commissioners met Gov. Reeder had been removed, and the commissions of John A. Halderman, as probate judge; Joseph M. Hall of Leavenworth, and Matthew R. Walker of Wyandotte, were signed by Daniel Woodson, the acting governor. The first meeting of the board was held on Sept. 7, 1856, at the warehouse of Lewis Rees in Leavenworth. James M. Lyle was appointed clerk of the board, and ex-officio recorder and clerk of the probate court. At this meeting the commissioners divided the county into Kickapoo, Delaware, Leavenworth, Alexandria and Wyandotte townships. They also appointed M. P. Rively, treasurer; L. T. Moore, assessor; Bennett Burnham, surveyor; Green D. Todd, sheriff; and James Blake, coroner. The constables and justices of the peace were appointed within a short time, and the county was thus permanently organized. No town had been named as the county seat and the board appointed A. Dawson, Martin Hefferlin and Samuel El. Burgess judges for an election to he held on the second Monday in October to decide upon a permanent location. Eight miles above Leavenworth was Kickapoo City, a strong pro-slavery settlement, and 6 miles down the river was Delaware City, a flourishing community, with many advantages to offer. The election was therefore a hard triangular fight for the location of the county seat. Pro-slavery voters were imported from Missouri to Kickapoo and Delaware. Kickapoo won, and when this was learned Delaware City opened the polls a second day, which resulted in the greatest number of votes being cast for Delaware. A legal contest followed with a decision in favor of Delaware. A county building of two rooms was erected and the county offices removed there on Feb. 20, 1857, but the triumph of the town was short lived, as the legislature of 1857 ordered a new election, at which Kickapoo City received a majority of votes cast. Leavenworth petitioned that the Kickapoo votes be cast out as illegal, on the ground that the voters had not been confined to the county. It was finally decided that Leavenworth received the greatest number of legal votes and was declared the county seat. Jeremiah Clark donated land for the court-house square, which was accepted. This was the east half of "Block 13." The county purchased the other half for $13,000, and $35,000 worth of county bonds were voted for the erection of county buildings. Notwithstanding this celerity, the county offices were located in the city hall for many years and it was not until 1873 that the court-house was erected. It is estimated that the entire cost of the county buildings when completed aggregated $175,000. In May, 1860, a tract of 200 acres of land was purchased about 4 miles southwest of Leavenworth for a county poor farm, on which a county house and contagious hospital were erected, the entire property being valued at over $12,000.

The first school districts were established in 1858, with George E. Budington as superintendent, but the first school in Leavenworth and also in the county is supposed to have been started in May, 1855, by Rev. J. B. McAfee, who purchased a small building near the levee for religious purposes. Here was taught the first public school in the county. In 1851 the Catholic diocese of Leavenworth was created as a mission. The first mass was said in the county in 1854. The Methodists held meetings in 1854. A church was built in 1857. In Jan., 1856, the Presbyterians organized at Leavenworth the first church of this denomination in Kansas. During the late '50s and early '60s other denominations followed. The Herald was the first newspaper in Leavenworth county and in the territory, the first number being issued Sept. 15, 1854. The state penitentiary is located 5 miles south of Leavenworth. (See Penitentiary, State.) The Federal military prison was built on the military reservation of Fort Leavenworth in 1874, and the Federal prison, for offenders against the laws of the United States is also located there. The National Home for Disabled Volunteer Soldiers (Federal) is situated on a beautiful site of ground south of the city of Leavenworth. In the vicinity of the home has grown up a considerable city called National Military Home.

At the opening of the Civil war both the city and county of Leavenworth were better populated than many of the towns and counties of Kansas which lay to the west, and it was but natural that many troops should be raised. Union men and their families who had resided in the border counties of Missouri came in considerable numbers to Leavenworth for protection and their numbers swelled the ranks of the volunteers. Many of the leaders who gained a name for bravery and courage during the great conflict called Leavenworth home. Cutler, in his History of Kansas, says: "By the 20th of May, 1861, eighteen companies were organized." After that date the following companies were raised: Kickapoo Guards, Black's Guards, Leavenworth Mercantile Guards, Leavenworth Light Cavalry, Lyon Guards, Third Ward Guards, Fourth Ward Guards, Old Guard and the Union Guards (cavalry). Leavenworth county was represented in many of the Kansas regiments and some of her bravest men were officers.

The Kansas Pacific railroad was started at Wyandotte in 1863 and completed to Denver within two years. Leavenworth became one of the terminals. This began a new era in railroad building and since that time a number of roads have been built in the county until today transportation and shipping facilities are provided by the main line of the Union Pacific. Three lines of the Atchison, Topeka & Santa Fe, a line of the Missouri Pacific from Kansas City, the Leavenworth, Kansas & Western, a branch of the Missouri Pacific which enters the county on the east and crosses west into Jefferson county, and the Chicago, Burlington & Quincy. The Kansas City & Leavenworth electric railway runs southeast from Leavenworth and connects the two cities. There are over 182 miles of main track railroad in the county.

In the spring of 1911 the court-house in Leavenworth burned and the county offices were temporarily located in business blocks until provision was made for a new county building. The population of the county in 1910 was 41,207.