On June 20, the following notable historical events related to Missouri have been recorded:

• 1820: The Missouri Compromise, which balanced the admission of Missouri as a slave state with Maine as a free state, was a significant topic of discussion around this time, though not specifically finalized on June 20. On this date, Missouri's territorial leaders were actively preparing for statehood, with debates over slavery intensifying in St. Louis and beyond.
• 1863: During the Civil War, Confederate guerrillas, including those led by William Quantrill, were active in Missouri. While no major battle is recorded for June 20 specifically, skirmishes and raids were common in Missouri during this period, contributing to the state's volatile role in the conflict.
• 1904: The St. Louis World's Fair, or Louisiana Purchase Exposition, was ongoing, with June 20 being a notable day for exhibitions and cultural displays. The fair highlighted Missouri's economic and cultural prominence, attracting global attention to St. Louis.