This study redesigns and evaluates the usability of the Letterboxd Android application using the User-Centered Design (UCD) approach to address interaction problems in a feature-rich, film-based social mobile app. The baseline diagnosis triangulated contextual interviews, Cognitive Walkthrough (CW), System Usability Scale (SUS), and User Experience Questionnaire (UEQ) with nine active users, generating 49 user insights that were synthesized into 27 pain points across seven core features. These findings were systematically translated into revised information architecture, improved user flows, wireframes, and high-fidelity prototypes guided by Shneiderman's Eight Golden Rules. Post-redesign validation combined screen-based prototype testing on prioritized user tasks and a redesign survey summarized into feature-level SUS scores. The workbook-based final task dataset shows that average task success improved significantly from 76.6% before redesign to 93.8% after redesign, while the average feature-level SUS score increased from 53.7 to 77.0. The largest gains were observed in the Profile feature, although Carousell, Review, Filter Search, and List Creation still showed notable interaction friction that requires further refinement. Overall, the study contributes an auditable UCD-based redesign workflow for film-based social mobile applications by clearly tracing how multimethod baseline evidence was transformed into prioritized design decisions and supported by descriptive post-redesign validation results and findings overall.