Although research suggests that mountain biking (MTB) crashes often involve falls onto compliant trail surfaces, MTB helmets are presently designed around rigid surface impact tests. This study describes the investigation of a laboratory method for evaluating MTB helmets in oblique impacts against a surrogate compliant MTB trail surface. A headform was subjected to drop tests onto compliant surfaces at local MTB trails. Matched tests were repeated in a laboratory setting to identify a surrogate surface that could replicate the impact response of the trail surfaces. A conventional MTB helmet was then subjected to oblique impact testing against the surrogate trail surface as well as a rigid surrogate road surface typically used in helmet testing. Generally, the surrogate trail surface produced decreased linear and rotational kinematics and longer durations compared to the surrogate road surface. However, these trends were dependent upon impact location, with higher peak rotational velocities observed for the trail surface at one location. Notably, predicted brain injury risks (Brain Injury Criteria, Abbreviated Injury Scale 2) were moderate (22-36%) across both surfaces. Researchers and manufacturers can use similar testing against compliant surfaces to evaluate previously unexplored aspects of MTB helmet impact performance.