External program manager involvement can provide professional and systematic management services to owners. It can help to manage and coordinate different subprojects and ensure the achievement of the overall strategic goals of large projects. The promotion of external program manager involvement is not only efficiency-driven, but the institutional environment also has a significant impact. This study developed a theoretical model to explain how external program manager involvement is influenced by external institutional forces (coercive, mimetic, and normative pressures) and internal top management support. Based on a questionnaire survey, this study tested the research model through partial least-squares structural equation modeling. The empirical results revealed that coercive pressures have no significant impact on external program manager involvement; owners’ top management support acts as a complete mediator between the mimetic pressures and external program manager involvement; normative pressures have the greatest impact on top management support and external program manager involvement, and top management support plays a key but partially mediating role. A direct contribution of this study is to reveal how different types of isomorphic pressures can be better exercised to facilitate the involvement of external program managers. Going further, by incorporating both external institutional pressures and internal top management support into the research model, this study provides insight into how organizations are influenced by external institutions to apply tools or approaches internally.