This article explores how Xamarin simplifies cross-platform mobile app development and highlights the importance of design patterns in tackling modern software challenges. As demand grows for adaptable, high-performing apps, Xamarin empowers developers to create seamless solutions for iOS, Android, and Windows using C#. The text introduces key design patterns like Model-View-ViewModel (MVVM), singleton, and dependency injection, showing how they reduce complexity, boost code reuse, and improve maintainability. It also discusses the transition to .NET MAUI, the next-generation framework built on Xamarin's foundation, offering enhanced performance and flexibility. A practical example demonstrates the Model-View-ViewModel (MVVM) pattern in a task list app, illustrating how design patterns solve real-world problems. By focusing on efficient resource management and scalable design, this article provides actionable insights to overcome challenges in cross-platform development, ensuring robust and maintainable applications.