Climate change is causing shifts in the growing seasons of plants1,2, affecting species performance and interactions3,4 as well as global carbon, water and nutrient cycles5,6. How the timing of autumn leaf senescence in extra-tropical forests will change remains unclear because of the complex seasonal interaction of climate warming, earlier and enhanced vegetation activity, and the annual day-length cycle7–10. Here we use experiments, long-term ground observations, and satellite-based vegetation monitoring to show that early-season and late-season warming have opposite effects on the onset and progression of leaf senescence, with a reversal occurring at the year’s longest day (summer solstice). Across ~84% of the northern forest area, increased temperature and vegetation activity before the solstice led to an earlier senescence onset (10% greenness loss) of, on average, –1.6 ± 0.1 days-per-°C, while warmer post-solstice temperatures did not affect senescence onset but reduced its speed (progression to 50% greenness loss) by 0.8 ± 0.1 days-per-°C. Due to the earlier senescence onset, the day at which autumn temperature starts driving senescence progression has been shifting to ever earlier dates, between 1951—2015 at a rate of –0.20 ± 0.07 days per year. These developmental constraints suggest that senescence will start earlier but progress more slowly in the future, revealing Northern Hemisphere-wide compensation effects on trends in growing-season length, caused by enhanced pre-solstice vegetation activity. This new mechanistic insight improves our ability to model carbon uptake by extra-tropical forests under climate change.