The combination of high nitrogen (N) inputs on tile-drained agricultural watersheds contributes to excessive nitrate (NO3-) loss to surface- and groundwater systems. This study combined water age modeling based on StorAge Selection functions and NO3- isotope analysis to examine the underlying mechanisms driving NO3- export in an intensively tile-drained mesoscale watershed typical of the U.S. Upper Midwest. The water age modeling revealed a pronounced inverse storage effect and a strong young water preference under high-flow conditions, emphasizing evolving water mixing behavior driven by groundwater fluctuation and tile drain activation. Integrating water age dynamics with isotope-discharge relationships observed at the watershed scale illuminated interactions between flow path variations and subsurface N cycling as key drivers of variable NO3- export regimes at the seasonal scale. Based on these results, a simple transit time-based and isotope-aided NO3- transport model was developed to estimate the timescales of watershed-scale NO3- reactive transport. Model results demonstrated a wetness dependence for denitrification at the watershed scale and further suggested that interannual NO3- export regimes are controlled by coupled and proportional responses of soil N cycling and hydrologic transport to varying wetness conditions. Although uncertainties remain, the estimated transport and reaction timescales suggest a relatively minor N legacy effect for intensively tile-drained agricultural watersheds in this region. Collectively, the results of this study demonstrate the potential of integrated water age modeling and NO3- isotope analysis to advance the understanding of macroscale principles governing coupled hydrologic and N biogeochemical functions in watershed systems.