This paper presents the design and flight test validation of a co-designed guidance and control system for a fixed-wing unmanned aerial vehicle (UAV). The co-design of all integrated control loops enables increased performance capabilities which are especially important for highly dynamic UAV missions. The control design is based on a mixed-sensitivity control approach to enable robust inner loop controls in the presence of uncertainties. The guidance design implements a look-ahead path following algorithm which enables superior performance metrics while considering the computational constraints present for small UAV systems. The co-designed guidance and control approach is applied to a small fixed-wing UAV and flight tested mimicking a highly dynamic urban flight trajectory. The flight-test results not only validate the seamless implementation of the developed algorithms on the available flightcontrol computer, but also demonstrate their robust performance during real-world operations.