Co-leadership is becoming a more popular and prominent leadership structure across a variety of industries and settings; however, the body of knowledge on this topic is limited. Much of the current research focuses on whether co-leadership is a beneficial or detrimental form of leadership and there exists a gap in our knowledge about how and when co-leadership is or is not successful. The current study aims to fill this gap by surveying a group of 48 U.S. Army co-leader dyads and their direct superiors and subordinates on antecedents and outcomes of this co-leader relationship. Results demonstrate that engaging in effective co-leader processes such as participative decision-making, willingness to learn from each other, and supporting and empowering each other was related to higher quality co-leader relationships. Additionally, participative decision-making and communication quality were related to co-leader effectiveness. Results also revealed that each member of the co-leader dyad may view these antecedents of relationship quality differently. Further, positive co-leader processes and interactions were related to key outcomes such as co-leader effectiveness and team effectiveness. Unexpectedly, co-leader relationship quality was not related to co-leader effectiveness nor team effectiveness and level of dyadic congruence on these antecedent processes was not related to relationship quality. Overall, these findings suggest that co-leadership can work and provide preliminary evidence of antecedents of success and failure.