The Missouri Compromise was principally engineered in Congress by Speaker of the House Henry Clay of Kentucky, who earned the nickname “the Great Compromiser” for his role in brokering it.
Senator Jesse B. Thomas of Illinois first proposed the key amendment drawing the 36°30′ line to limit the expansion of slavery, but Clay took the lead in steering the overall package—pairing Missouri’s admission as a slave state with Maine’s as a free state—through the House and helping secure final passage.
President James Monroe then signed the Missouri Compromise into law on March 6, 1820, formalizing the agreement reached under Clay’s leadership.
