Paired measurements of ¹⁴C/¹²C and ²³⁰Th ages from two Hulu Cave stalagmites complete a precise record of atmospheric ¹⁴C covering the full range of the ¹⁴C dating method (~54,000 years). Over the last glacial period, atmospheric ¹⁴C/¹²C ranges from values similar to modern values to values 1.70 times higher (42,000 to 39,000 years ago). The latter correspond to ¹⁴C ages 5200 years less than calibrated ages and correlate with the Laschamp geomagnetic excursion followed by Heinrich Stadial 4. Millennial-scale variations are largely attributable to Earth’s magnetic field changes and in part to climate-related changes in the oceanic carbon cycle. A progressive shift to lower ¹⁴C/¹²C values between 25,000 and 11,000 years ago is likely related, in part, to progressively increasing ocean ventilation rates.