Introduction: Being aware of and willing to access mental health services are important first steps in help-seeking behaviour. However, evidence suggests that UK armed forces personnel are not always aware of or willing to access sources of mental health support. This study explored which sources of support UK armed forces personnel are most aware of, and willing to use, for a self-reported mental health problem and the possible differences between serving and ex-serving personnel. Methods: Data were taken from a cross-sectional study of 1,432 UK serving and ex-serving personnel who had self-reported a mental health, stress, or emotional problem in the past three years. Results: Military personnel, irrespective of serving status, were most aware of, and willing to access, formal medical services. In contrast, there was a low awareness of and willingness to use ex-serving-specific support services among ex-serving personnel. Discussion: Future service delivery and policy should focus on improving the variety of sources of support that ex-serving personnel are aware of, and willing to use, to enable them to make informed choices about where to seek help if needed.