The “15-minute city” concept provides a framework for livable, sustainable cities but often overlooks human mobility, public transit, and factors beyond proximity when measuring accessibility. This study introduces an interpretable, individual-level, trajectory-based accessibility measure considering connections to amenities through the transportation system, affordability, and operating hours in addition to proximity. A case study is performed in Stockholm, Sweden. Using ST-DBSCAN, travel patterns are mined from transit smartcard data of frequent transit users to build representative trajectories. Grocery store accessibility is defined as the number of stores reachable within a 15-walk of anchors along individual trajectories. Results show residents of the dense, walkable city center have high home-based accessibility, but fewer affordable stores. Considering mobility, individuals living in rural or suburban areas with high transit connectivity observe large, positive accessibility increases. However, those living in areas with poor transit connectivity observe marginal accessibility improvements. Residents of the city center also observe marginal accessibility improvements as they already live in resource-rich areas. The trajectory-based accessibility measure can identify areas requiring accessibility improvements or evaluate policy effects (e.g., changes in transit routes) on accessibility.