Jennings was born on March 27, 1784, in Readington, New Jersey, the son of Jacob Jennings and Mary (Kennedy) Jennings. His father had served as a surgeon in the Revolution and continued to practice medicine after he became an itinerant Presbyterian minister.
While Jonathan was a small boy the family moved to Fayette County, Pennsylvania, where he received his elementary schooling. Later he attended a grammar school at Canonsburg, Washington County, Pennsylvania.
In 1806 Jennings decided to migrate to the Northwest Territory. Embarking at Pittsburgh he went down the Ohio to Jeffersonville. Shortly after his arrival he was admitted to the bar and began practising law. Dissatisfied with this location, a year later, he proceeded to Vincennes, where he found employment as clerk in the territorial land office under Nathaniel Ewing.
In 1809 he left Knox County to take up his residence in Clark County. In that year he became a candidate for territorial representative to Congress on the platform of "no slavery in Indiana" and won the close election held on May 22. In 1811 and again in 1813 he was re-elected territorial delegate. After the passage of the enabling act on April 19, 1816, a constitutional convention met at Corydon, elected him as president, and drafted Indiana's first constitution within the short time of nineteen days. On Aug. 5 he was elected governor against Thomas Posey.
In 1818, along with Lewis Cass and Benjamin Parke, Jennings negotiated the St. Marys treaties of cession with the Potawatomi, the Wea, the Miami, and the Delaware. His enemies contended that the Governor had vacated his office as chief executive of the state when he served as commissioner for those treaties, but the state legislature refused to institute impeachment proceedings against him. In 1819 he was re-elected but, in 1822, resigned in order to run for representative to Congress from the second congressional district. He was successful and continued to hold the same office until 1830, when he was defeated. In 1832, he again served as commissioner, with Marks Crume and John W. Davis, to conclude land cessions from several bands of the Potawatomi.
In his last years he became involved in financial difficulties and was only saved from actual want by the generosity of loyal friends. He died on his farm and was buried in a country cemetery in Charlestown, Indiana.
Jennings was given high praise and credited with the defeat of the pro-slavery forces in Indiana and with laying the foundation of the state. On the other hand, he is sometimes described as a crafty and self-promoting politician with drinking addiction. Some modern historians place Jennings's importance between the two extremes, but say that the state "owes him more than she can compute. "
Jennings was a member of the U. S. House of Representatives from Indiana's 1st district and a delegate to the U. S. House of Representatives from Indiana Territory.
Jennings had been a heavy drinker of alcohol for much of his life. His addiction worsened after the death of his first wife, Ann, and his development of rheumatism. Jennings's alcoholism led to defeat in his reelection campaign in 1830. In retirement his condition worsened and he was unable to work his farm.
In 1811 Jennings married Ann Gilmore Hay, the daughter of John Hay of Clark County, who accompanied her husband on horseback through fifteen hundred miles of wilderness to Washington. Having lost his first wife in March 1826 he had married, on October 19, 1827, Clarissa Barbee of Paducah, Kentucky, with whom he retired to his farm near Charlestown after 1830.
