The seasonal cycle of vertical mixing is crucial for lake ecosystems, yet its future under climate change remains uncertain. While lake stratification shifts have been widely studied, the annual overturning duration changes are less clear. Using sub-daily simulations from a fully coupled numerical Earth system model, we assess phenological changes in stratification and overturning in Northern Hemisphere ice-covered lakes. We find the total stratification duration (comprising both summer and winter phases) is projected to decrease by 0.7, 4.6, and 6.9 days in 2029, 2067, and 2096, respectively, under global temperature increases of 1.5 °C, 3 °C, and 4.5 °C. Conversely, the duration of overturning is expected to increase by 0.7, 4.2, and 8 days annually. Notably, these changes are asymmetrical, with most of the overturning extension occurring in the fall, following the peak growing season. This extended overturning could affect lake ecosystems, particularly through enhanced ventilation of bottom layers and altered nutrient cycling.