The Trans-Himalaya, located between the Great Himalaya and the Tibetan Plateau, is ecologically fragile and one of the most climate-sensitive regions. However, lack of systematic in-situ measurements have hindered the evaluation of chemistry-climate models and constrained our understanding of the global-to-regional influences over this region. To address this, we present a comprehensive analysis by combining first yearlong (October 2024–September 2025) surface ozone (O₃) measurements at Leh (3.4 km above mean sea level) with satellite data and numerical simulations (global model: CAM4-Chem, regional model: WRF-Chem). Our measurements revealed a noontime O₃ build-up (~3–7 ppbv), a feature typically absent over high altitude mountains. WRF-Chem suggests that this feature is driven by photochemical production (NO_x-VOC transition regime for higher O₃) and entrainment. Despite partial compensation of nighttime loss by noontime build-up, the region acts as a net chemical sink. Seasonal cycle peaks in June (~64±6 ppbv; touching air quality threshold), unlike typical premonsoon peaks over Gangatic and Himalayan regions. CAM4-Chem simulation with CAMS-derived stratospheric contribution, captured the O₃ seasonality, but overestimated the wintertime levels by 10–16 ppbv. Tagged O₃-tracer analysis shows significant contributions of anthropogenic NO_x emissions from South Asia (15–18 %) and other regions (32–37 %) to O₃ levels. Additionally, stratospheric inputs and biogenic emissions shape the seasonal cycle. This study provides insight into the chemistry and dynamics governing O₃ over the Trans-Himalaya across different time scales. These findings are invaluable for designing future field observations, model improvements, and policy planning for this region.