In an external magnetic field, the energy of the electroweak sphaleron—representing the energy barrier to baryon and lepton number violation—decreases but remains nonzero until the upper Ambjørn–Olesen critical field strength set by the Higgs mass and the electric charge. At this point the sphaleron energy vanishes. We demonstrate this by numerically computing the sphaleron configuration in the presence of an external magnetic field over the full range of field strengths until the energy barrier vanishes. We discuss the implications for baryogenesis in the early universe and the possibility of observing of baryon and lepton number violation in heavy-ion collisions.