From Citizendium, the Citizens' Compendium
The term originated from John Hancock around 1781 to describe a group of inter-related Essex County, Massachusetts, politicians and merchants who strongly favored a strong national government. By the 1790s, after the ratification of the U.S. Constitution the term fell into disuse as those men joined the Federalist Party and became staunch supporters of Alexander Hamilton.
Members of the Essex Junto included Samuel Cabot, George Cabot, John Cabot, Francis Cabot Lowell, Thomas Pickering, Theophilus Parsons, Stephen Higginson, Benjamin Goodhue, and Fisher Ames. As they supported Hamilton, they were not supportive of John Adams. Adams revived the term in 1798, blaming the "junto" for attempting to foist a war with France upon the United States (see Quasi War).
By Thomas Jefferson's administration, the term became less associated with local politics and became more synonymous with New England Federalism. The Embargo of 1807 greatly infuriated these men. They saw no cause for war 1810-1812, opposed the War of 1812, and organized the Hartford Convention with its scarcely veiled threat of secession. The ill-timing of the Hartford Convention, however, was their (as well as the Federalist Party's) undoing.
Alexander Johnston, "Essex Junto," Cyclopædia of Political Science, Political Economy, and the Political History of the United States by the Best American and European Writers, (New York: Maynard, Merrill, and Co., 1899).