Improving access to HIV testing among youth at high risk is essential for reaching those who are most at risk for HIV and least likely to access health care services. This study evaluates the usability of mLab, an app with image-processing feature that analyzes photos of OraQuick HIV self-tests and provides real-time, personalized feedback. mLab includes HIV prevention information, testing reminders, and instructions. It was developed through iterative feedback with a youth advisory board (N = 8). The final design underwent heuristic (N = 5) and end-user testing (N = 20). Experts rated mLab following Nielsen's heuristic checklist. End-users used the Health Information Technology Usability Evaluation Scale. While there were some usability problems, overall study participants found mLab useful and user-friendly. This study provides important insights into using a mobile app with imaging for interpreting HIV test results with the goal of improving HIV testing and prevention in populations at high risk.