Mountain regions are being affected by climate warming at a rate up to twice the global average, making them among the most rapidly changing ecosystems. These climatic shifts, along with altered precipitation regimes and increased nitrogen deposition, have rendered mountain ecosystems particularly vulnerable and important as early indicators of global change impacts. To improve mechanistic understanding of how environmental drivers influence mountain vegetation and ecosystem processes, a systematic review of three decades of manipulation experiments was conducted. Seven major global change drivers were assessed: temperature, water availability, nutrient addition, snow manipulation, radiation, atmospheric gases, and disturbance. Temperature was the most frequently manipulated driver (in 45% of studies), followed by nutrient addition (15%) and water availability (14%). Across 767 studies, consistent effects of warming were observed on plant life history traits, functional characteristics, and phenology. Phenological events were generally accelerated, and shifts in species composition were commonly recorded under experimental warming. Direct impacts on plant development and ecosystem function were also reported in relation to changes in water and nutrient availability. Soil microbial communities were found to respond rapidly to warming, with consequences for nutrient cycling and decomposition. Long-term studies indicated that interactions between warming and soil-vegetation feedbacks influence carbon and nutrient dynamics. In studies where temperature and water availability were manipulated together, soil moisture was shown to mediate the effects of warming on productivity and biogeochemical processes. However, biotic interactions were rarely studied (only 2% of responses), despite evidence suggesting that warming may disrupt plant–pollinator relationships and alter competitive dynamics. Although vegetation responses to global change drivers were well documented, major research gaps were identified—particularly in tropical and boreal mountains and in understanding adult tree responses. Integrated approaches combining long-term monitoring with multifactor experiments were recommended for future research.