*** THIS QUESTION IS A FAQ - this top post will be used to provide the best info from below ***
There are a TON of events that claim to be "the longest running". Fortunately we can just compare dates.
, Natural Chimneys, (Since at least 1840)
There’s not a ton of great documentation floating around out there to corroborate the oral tradition, though. The tale of the inaugural 1821 joust has been rewritten time and again in local newspapers and other accounts, and a 1941 article in the Maryland Historical Magazine mentions the yearly tournament at Natural Chimneys as one of the best-known jousts in the South. Nevertheless, the article identifies an 1840 joust outside of Baltimore as the first to definitively occur in the United States.
, 1851 -- except, it's not always been raced in the USA.
Intercollegiate boat races
(rowing) first held between Harvard and Yale in 1852. NOTE: First contested in 1852, annually since 1859 except during major wars fought by the United States.
(1875) -- lots of claims that this is the oldest, but it doesn't appear to be unless you disqualify those listed above.