Fermat's Last Theorem, first conjectured by Pierre de Fermat around 1637, was not proven until 1994 by Andrew Wiles.
Notes on verification
Well-documented historical mathematical fact confirmed by Wikipedia, Scientific American, and Cambridge University sources. Minor nuance: initial 1993 announcement had a flaw, corrected proof completed in 1994. [tier=bronze indep_score=0.3 clusters=2 claim_tier=everyday]