Vegetation phenology increasingly shapes regional land–atmosphere interactions in ways that matter for both climate science and climate-sensitive land management. Using long-term remote-sensing observations and fully coupled Weather Research and Forecasting simulations, this study quantifies how observed phenological shifts between the early 2000s and the late 2010s altered land-surface temperature across Northeast China, a major grain-producing region. The analysis focuses on the two dominant land-cover systems—temperate forests and croplands—and jointly evaluates timing shifts, seasonal thermal responses, and the biophysical pathways through which canopy change affected surface temperature. The results show that forests experienced earlier spring development, while croplands combined earlier green-up with delayed dormancy, producing a larger extension of the growing season in managed landscapes. The paired simulations indicate that these changes increased canopy development and strengthened vegetation–climate coupling, yielding more pronounced and more persistent cooling in croplands than in forests. Forest cooling was concentrated in May during rapid green-up, whereas cropland cooling was strongest in June, July, and September. Biophysical attribution indicates that reduced aerodynamic resistance was the dominant cooling pathway, with atmospheric feedback as the second most important contributor. Taken together, the findings establish that phenology is a material planning variable: in regions where agricultural calendars, land management, and climatic risk interact closely, shifts in canopy timing can measurably alter seasonal thermal conditions and should be incorporated into climate adaptation and land-use planning.