Leaders at higher organizational levels, like lieutenant colonels in the military and senior executive club directors in elite team sports organizations, are two underresearched leader-level-specific populations. They operate in two contexts that differ in their focus on military operations versus sporting events. However, their similarities include a shared overarching responsibility for organizational objectives, training, preparation, and the performance optimization of their staff, teams, and units to ultimately win against opponents in physically and mentally stressful circumstances. In such conditions, leadership is essential for success. Addressing similarities while being aware of the differences has inspired a psychological cross-contextual leadership research approach. This aligns with the call for increased mutual research between the military and sport. We need a more profound understanding of how high-level military and sportleaders respond to tough situations and their inherent leadership challenges. Examining how those circumstances affect the leaders’ performance and mental health, and what psychological skills they use to enhance their performance, provides valuable information to leader development initiatives. To further inform such practice, it is valuable to study leader development programs. Hence, this thesis investigates high-level leaders in military and sport using a psychological cross-contextual leadership research approach. This approach means examining leadership from a psychological lens among two or more populations operating in their respective distinct contexts, with the aim of comparing and integrating the results to identify common patterns. Two initial qualitative empirical studies anda subsequent quantitative empirical study employ this approach, followed by a final scoping review on coaching-based executive leader development programs. The overriding aim was to improve knowledge of the similarities between high-level military and sport leaders’ performance, psychological skills, and mental health under demanding conditions. Additionally, the thesis aimed to enhance the understanding of coaching-based executive leader development programs and their encompassing of health. More specifically, Study I aimed to simultaneously explore high-level military and sport leaders’ real-life experiences of similarities in demanding conditions and their psychological manifestations. The results revealed four common themes of demanding conditions. They were briefly labeled as developing organizations, dealing with destructive superiors and subordinates, taking care of others' moods and minds, and periods of extreme concentration. These conditions were mainly manifested by high cognitive, emotional, and physical stress reactions. Study II aimed to explore similarities in the performance of leadership behaviors and the application of psychological skills under demanding conditions. A cognitive–behavioral thematic structure model was generated and illustrated a dynamic setup of functional–purposeful leadership behaviors, as well as a combined set of psychological skills. To balance the initial qualitative studies, Study III, a longitudinal questionnaire study, investigated how demanding conditions were related to self-rated leader performance level and mental health indicators. Furthermore, the study explored whether the use of psychological skills influenced these relationships. The study revealed that demanding conditions had no effect on leader performance. However, they had a negative impact on the leaders’ vitality and were associated with higher stress symptoms. As an integrated psychological skill for emotion regulation, however, mindfulness and cognitive reappraisal had a positive impact on vitality. Based on the findings from Studies I-III, the final Study IV was a scoping review aimed at exploring the characteristics and emerging key findings of coaching-based executive leader development programs and their encompassing of health. The findings revealed that these programs rarely include health. However, a few principal discoveries indicated that a focus on health can enhance leaders’ wellbeing. These findings pave the way for the overt inclusion of health in similar interventions. In addition, a combination of individual coaching and group coaching is advised. In summary, the current thesis improves our understanding of high-level military and sport leaders’ performance, psychological skills, and mental health under demanding conditions. The similarities identified in these areas promote further psychological cross-contextual leadership research, which corresponds with the call for mutual research between the military and sport. Furthermore, the thesis provides insights into coaching-based executive leader development programs and their integration of health. Altogether, this thesis provides guidance for cognitive-behavioral leader-development initiatives to support leaders inimproving both their wellbeing and performance under demanding conditions.