Effective leadership, especially from executive leaders, is a key factor in successful mergers and acquisitions (M&A). During M&As, the volume of work increases significantly for executive leaders, and competing priorities often cause executive leaders to regularly work beyond the standard 40-hour week. This may lead to stress and overwork, which may have a negative impact on leader efficacy. The condition of overwork involves working excessively long hours, regularly beyond the standard 40-hour work week without adequate rest or recovery. The purpose of this research study was to gain a deep understanding and contribute insights into the lived experience of overwork from the perspective of executive leaders engaged in M&A. This mixed methods study explored the perceptions of 24 executive leaders from publicly traded companies regarding their experiences with overwork during M&A. Data were collected from interviews, the Leader Efficacy Questionnaire (LEQ), and the Maslach Burnout Inventory (MBI); data were analyzed using thematic analysis, statistical tests, and comparison. Three key findings emerged from this exploratory study. Leading M&A projects is a highly stressful experience that results in overwork, which then leads to a decrease in emotional and physical well-being for leaders. Contradiction in reports show quantitative results indicating that leaders are not yet burned out, but qualitative results indicate that they are physically and emotionally drained, and their well-being suffers during periods of M&A. Conditions of M&A environments make leaders less effective, and overwork negatively impacts their confidence, or leader efficacy. This study extends awareness of the impact overwork has on the well-being of leaders working excessive hours and contributes to an understanding of how overwork influences leader efficacy during times of M&A. The findings suggest that companies need to provide more support for executive leaders who are executing M&A projects in addition to their existing work to help relieve the conditions of overwork and their sources of stress. Keywords: overwork, occupational stress, burnout, leadership effectiveness, leader efficacy, leadership development, mergers and acquisitions (M&A)