Born in Sandwich, New Hampshire, John Wentworth was a huge man, towering 6 ft 6 in (1.98 m) high and weighing more than 300 pounds (136 kg). He drank at least a pint of whiskey each day and would eat from 30-40 different foods during a single meal.
Wentworth was educated at the New Hampton Literary Institute and Dartmouth College graduating from the latter in 1836. Later that year, he left for Chicago, arriving in the city on October 25. He was managing editor of Chicago's first newspaper, the Chicago Democrat, eventually becoming its owner and publisher. He started a law practice, and eventually entered politics. In 1844, he married Roxanna Marie Loomis.
He served for six terms in the U.S. House of Representatives (March 4, 1843 – March 3, 1851 and March 4, 1853 – March 3, 1855 as a Democrat; and March 4, 1865 – March 3, 1867 as a Republican). While in the House, a controversial vote arose by which Wisconsin claimed land in Illinois as far as the tip of Lake Michigan. If Wentworth voted to give the land, including Chicago, to Wisconsin, he was promised a Senate seat. Wentworth declined the offer.
As a Republican, Wentworth served as mayor of Chicago for two terms, 1857–1858 and 1860–1861. Wentworth instituted chain gangs in the city and tried to clean up the city's morals. To do this, he hired spies to determine who was frequenting Chicago's brothels. In 1857, Wentworth led a raid on the Sands, Chicago's red-light district, which resulted in the burning of the area.
Wentworth was a personal friend of Abraham Lincoln.
He authored The Wentworth Genealogy - English and American. 1878, 3 Volumes, 2241 pages. The first volume chronicles the ancestry of Elder William Wentworth, the first of this family in New England, and his first five generations of New World descendants. The second and third volumes discuss the "Elder's" many descendants and others of the name. (Chicago Giant: A Biography of "Long John" Wentworth. By Don E. Fehrenbacher. (Madison, Wis.: American History Research Center. 1957. Pp. viii, 278.))
From 1868, he lived at his country estate at 5441 South Harlem Avenue in Chicago where he owned about of land in what is today part of the Chicago neighborhood of Garfield Ridge and suburban Summit. Wentworth died at the estate in 1888, and was buried in Rosehill Cemetery. The house was then sold and lived in by another family for several decades until it was torn down in the mid-1960s to make way for new single family housing as the Garfield Ridge neighborhood exploded in population.