Early life and political career
McClernand was born in Breckinridge County, Kentucky, near Hardinsburg, but his family moved to Shawneetown, Illinois, when he was quite young. His early life and career were similar to that of another Illinois lawyer of the time, Abraham Lincoln. He was largely self-educated and was admitted to the Illinois bar in 1832. In that same year he served as a volunteer private in the Blackhawk War (Lincoln briefly served as a captain).
In 1835 McClernand founded the Shawneetown Democrat newspaper, which he edited. As a Democrat he served in 1836 and from 1840 to 1843 in the Illinois House of Representatives.
He served in the United States House of Representatives from 1843 until 1851. He was known for his bombastic oratory and his adherence to Jacksonian principles. His dislike of abolitionists generated favor among his constituents, many of whom were originally natives of slaveholding states, as he was. McClernand vigorously opposed the Wilmot Proviso. He was an important ally to Illinois Senator Stephen A. Douglas. Douglas played a crucial role in formulating the Compromise of 1850, and McClernand served as a liaison for him the House of Representatives during the debate over the proposed compromise. McClernand also served as Chairman of the Committee on Public Lands from 1845 to 1847 and on the Committee on Foreign Affairs from 1849 to 1851. In 1850, McClernand declined to be a candidate for renomination, and his term expired in 1851.
McClernand was again elected to the House to fill a vacancy caused by the death of Thomas L. Harris. His term began on November 8. He was a strong Unionist and introduced the resolution of July 15, 1861, pledging money and men to the national government. In 1860 he was defeated in a bid for the speakership of the House of Representatives; the coalition of representatives opposing him objected to his moderate views on slavery and the importance of retaining the Union.
McClernand supported the campaign of his friend, Stephen Douglas, in the 1860 presidential election. He served as one of his campaign managers during the divisive Democratic presidential nomination convention held in Charleston, South Carolina in 1860.
In November 1842, McClernand married Sarah Dunlap of Jacksonville, Illinois, a close friend of Mary Todd Lincoln. Sarah was a daughter of
James Dunlap, who served as a quartermaster in the Union Army during the Civil War, eventually appointed to the rank of brevet major general. John and Sarah's son, Edward John McClernand, was notable as a U.S. Army brigadier general in the Indian Wars and later in the Philippines. After Sarah's death, McClernand married her sister, Minerva Dunlap.