The hospitality industry's reliance on manual, paper-based reservation processes creates significant operational challenges for small-to-medium hotels, including room overbooking, inconsistent billing calculations, vulnerable record-keeping, and the absence of 24-hour remote access. This paper presents the design and implementation of an automated Hotel Reservation Management System developed for Hybrid International Hotel and Suites, Elele Auchi, using the MERN (MongoDB, Express.js, React.js, Node.js) full-stack architecture. The system integrates the Paystack payment gateway for secure online transactions and the Brevo transactional email API to automatically deliver booking confirmations, check-in and check-out details, and payment receipts to guests. Following an Iterative Waterfall software development methodology, the system was implemented as a Three-Tier architecture separating presentation, application logic, and data layers. Performance was evaluated through fifteen structured test cases spanning normal operation, boundary conditions, and error handling, alongside a formal assessment against eight non-functional requirements guided by the Technology Acceptance Model. All test cases passed, confirming that the system eliminates manual billing errors, prevents concurrent double-booking, and provides administrators with a real-time analytics dashboard. The system demonstrates a scalable, cost-effective framework for digital transformation in small-scale Nigerian hospitality enterprises.