A single hardcoded API key. A forgotten debug log. A token cached in AsyncStorage instead of the keychain. Any one of these — in an app that took six months to build — can hand an attacker the keys to your users' data on day one of launch. Mobile is now the dominant attack surface. Industry analysts project mobile-targeted attacks will grow more than 40% by the end of 2026 , driven by the explosion of fintech, health, and AI-assistant apps that hold sensitive data on-device. And yet the gap between "we shipped" and "we shipped securely" has never been wider, especially for teams using AI app builders, no-code tools, or React Native templates that abstract the underlying platform away. This guide is a pragmatic walkthrough of the mobile app security best practices that actually move the needle — ordered roughly by impact-per-hour-of-work, grounded in the OWASP Mobile Application Security Verification Standard (MASVS), and written specifically with React Native and Expo developers in mind.…