Abstract The Atlantic Multidecadal Variability (AMV) influences North Atlantic sea surface temperatures and regional climate phenomena such as European droughts and hurricane activity. Its mechanisms remain complex due to long timescales and ocean‐atmosphere interactions. Here, we analyze a 2000‐year pre‐industrial climate model simulation revealing that millennial‐scale variability modulates AMV characteristics. During warm phases, the AMV is driven by a strong Atlantic meridional overturning circulation (AMOC) and subsurface warming, indicating ocean circulation dominance. In cold phases, the AMV arises from atmospheric heat fluxes with surface‐confined warming and AMOC lagging AMV. These results demonstrate that millennial‐scale ocean variability influences AMV mechanisms and the AMV‐AMOC relationship, providing insights into North Atlantic climate variability and its potential predictability.