Jennison was born in Jefferson County, New York, on June 6, 1834. He was educated in public schools until he was 12 years old, when his parents went to Wisconsin. At the age of 19, he began to study medicine. After completing his medical studies, he practiced for a short time in Wisconsin and then came to Kansas, settling at Osawatomie in 1857.
Within a short time, he moved to Mound City, where he remained for three years, and then to Leavenworth. Dr. Jennison was one of John Brown’s staunch supporters. Governor Charles Robinson commissioned him as captain of the Mound City Guards on February 19, 1861. On September 4, he was commissioned as a lieutenant colonel of the Seventh Kansas Cavalry, known as “Jennison’s Jayhawkers.”
Jennison’s troops, who wore red breeches, were also referred to as “Redlegs” and became one of the most notorious bands of Jayhawkers during the Civil War. He was assigned command of the western border of Missouri, with headquarters in Kansas City.
He determined to clear the border of guerrillas, and his name soon became a terror to lawless bands. His conduct was such that General Hunter appointed him acting Brigadier-General, and he was placed in command of “all the troops in Kansas west of and on the Neosho River.” At the time of the Lawrence Massacre, Governor Carney called upon Jennison to raise a regiment, of which he was made colonel on October 17, 1863, with headquarters in Leavenworth. While in command at Fort Leavenworth, he was authorized on March 5, 1864, to raise and organize a post battery. On July 20, 1864, he was placed in command of a regiment in the field and had command of the District of Southern Kansas.
He made a foray into Platte and Clay Counties, Missouri, against bushwhackers committing depredations in Kansas during the summer. In other ways, he successfully protected the border until Price’s Raid. At the time of this raid, he met Price’s forces at Lexington, Missouri, while reconnoitering under orders from General Curtis. With his regiment, he took part in the engagement at the Little Blue, where he was in command of the first division.
In the fall of 1864, he was elected as a member of the Leavenworth Council, was made president of the council, and was ex-officio mayor. In 1865 he was elected to the legislature from Leavenworth County; he was re-elected in 1867, and in 1872 was elected to the Kansas State Senate.
He died at Leavenworth on June 21, 1884.