To pursue radical innovation in emergent digital ecosystems, middle managers need to balance the often conflicting expectations and interests of internal and external stakeholders. Our longitudinal field study of Atos, a leading IT company, follows a team of middle managers aiming to collaborate with novel financial technology firms—so-called fintechs—to catalyze radical innovation. We explore how they navigated contradictory legitimacy issues arising from stakeholders both internal and external to the firm. In doing so, we outline how they adopted and adjusted a changing mix of legitimacy-seeking behavior over time to generate and sustain an array of radical innovations. Our findings show how legitimacy issues continuously evolve as middle managers embed their innovation efforts, driven by shifting stakeholder expectations, leading them to adjust their legitimacy-seeking approach. We contribute to the literature on collaborative innovation ecosystem strategies, emphasizing the central role of middle managers in connecting the internal organizational stakeholders to external ecosystem actors.