Background: The widespread prevalence of cancer across the globe demands cutting-edge solutions for its treatment. Current cancer therapies, notably chemotherapy, pose challenges due to their side effects. The early detection and management of the side effects are vital but complex. This study introduces a mobile health app designed to bridge the communication gaps between patients with cancer and health care providers. Hence, it allows patients to report symptoms immediately and also enables proactive symptom management by health care providers. Objective: This study has 2 objectives: first, to design a cancer-focused mobile health app that integrates educational content and real-time symptom reporting for chemotherapy patients. Second, to validate and evaluate the app quality using the Mobile App Rating Scale (MARS). The app seeks to foster health care communication, reduce hospital readmissions, and optimize symptom management, contributing to a more impactful patient experience. Methods: This mixed-methods study details the development and validation of mobile health applications. The app was designed by a multidisciplinary team, including nurses, medical professionals, pharmaceutical chemists, computer engineers, and software developers, using agile methodologies. For validation, the app was assessed by 13 evaluators, including clinical professionals (nurses and physicians) and engineers. The evaluation included technical performance analysis using Google tools and quality assessment using the MARS, which measures engagement, functionality, aesthetics, and information quality. Results: Performance metrics highlighted areas for improvement, with loading times showing delays in displaying content. Meanwhile, the response time of the app was moderate, and visual stability remained excellent. The app achieved an overall MARS score of 3.75 (SD 0.42), indicating consistent quality, with functionality scoring the highest (4.35; SD 0.52) and engagement the lowest (3.31; SD 0.61). The reliability of the MARS was confirmed (interclass correlation coefficient: 0.84; 95% CI: 0.72-0.92). Evaluators unanimously praised the app's potential benefits for patients and clinical professionals while identifying areas for improvement such as customization, onboarding guidance, and navigation. Conclusions: The CONTIGO app showed strengths in functionality, usability, and information quality, supported by robust security measures. However, areas such as user interactivity and engagement require improvement. Future refinements will integrate insights from patients with cancer to address user-specific needs and enhance the oncology care experience.