Early life and Bleeding Kansas
Charles R. Jennison was born on June 6, 1834 in New York. His family moved to Wisconsin in 1846 where Jennison studied medicine. Marrying at the age of 20, Dr. Jennison moved to Osawatomie, Kansas in 1858 and to Mound City shortly thereafter. Jennison was considered the most brutal and unscrupulous of the Jayhawkers. Whereas some other prominent leaders of irregulars in the Bleeding Kansas border conflict shared these traits, Jennison was distinguished by his blatant plunder for personal gain. Jennison cooperated with James Montgomery in opposing pro-slavery settlers and irregulars. In command of nine men, Jennison "tried" and hanged Russell Hinds near the state line at Mine Creek for the offense of returning a fugitive slave to his master in Missouri. Returning a slave was not only legal, but required at the time under the Fugitive Slave Act.
 Civil War
Even before the start of hostilities Jennison became a captain of the Mound City Guards on February 19, 1861.  Although not with Senator James H. Lane's Kansas brigade during the Sacking of Osceola, Jennison was associated with it and would soon join the fray after receiving a commission as colonel from Kansas Governor Charles L. Robinson on September 4.
On October 28, 1861, Jennison completed the organization and mustering of his 7th Kansas Cavalry. The regiment would become known as "Jennison's Jayhawkers." It immediately took to the field patrolling the Kansas-Missouri border to prevent the secessionist Missouri State Guard under Sterling Price from crossing. While the regiment was at Leavenworth, Kansas in April 1862, Jennison, angered over James G. Blunt being named brigadier general in his stead, resigned from the army and turned to banditry as a Redleg.
Following the Lawrence Massacre Jennison was once again commissioned a colonel and called into service by Kansas Governor Thomas Carney to raise a regiment that would become the 15th Kansas Cavalry on October 17, 1863.
Colonel Jennison commanded a mixed brigade of Kansas militia and volunteers resisting Price's Raid in October 1864. However, in December he found himself under arrest as the result of plundering while returning through Missouri after the pursuit of Price. Jennison was finally court-martialled for some of his many misdeeds and convicted on June 23, 1865, whereupon he was dishonorably dismissed from the service.