This article reconciles mixed findings about the performance impact of middle managers' strategy involvement. We propose that the relationship between middle managers' adaptive strategy implementation—through upward and downward influence—and objective business performance can be curvilinear and contingent on formal and informal structures. Applying a multilevel perspective to social networks, we empirically show that reputational social capital enhances the performance impact of middle managers' upward influence while informational social capital elevates the performance impact of their downward influence. The size of a business unit or region has differential moderating effects. The curvilinear effects of middle managers' upward influence and reputational and informational social capital on business unit performance reflect paradoxes. We discuss the implications of these findings for strategy implementation research and practice. Copyright © 2013 John Wiley & Sons, Ltd.