The deep winter mixed layer in the Kuroshio Extension (KE) region is important for the climate and ecosystems, yet its spatiotemporal variations and ecological impacts under pronounced ocean warming remain uncertain. Based on multiple observational and reanalysis datasets from 1980 to 2025, we find a pronounced shoaling (~40 m) of the mixed layer north of the KE in March, a reduction notably larger than in other months. Since the early 2010s, the annual deepest mixed layer depth (MLD) north of the KE has shifted from March to February, coinciding with a one-month earlier phytoplankton bloom. The weakened and poleward-shifted Aleutian Low in March is crucial to drive this seasonal advance. In the southern KE, MLD remains overall unchanged, because the Aleutian-Low induced shoaling is offset by opposing ocean dynamical effects. These findings suggest an emergence of marine phenological shifts north of the KE.