["Personnel Psychology, EarlyView. ", "\nABSTRACT\nNew ventures face ongoing demands for exploratory and exploitative activities (i.e., explore new opportunities while also optimizing their existing operations) throughout their establishment and operations. To succeed, entrepreneurial teams must therefore develop and sustain team ambidexterity—the ability to integrate these competing demands—over time. Given that entrepreneurial team formation has a profound and enduring influence on venture growth, this research examines how specific formation strategies shape the ongoing development of team ambidexterity and entrepreneurial effectiveness. Drawing on the dynamic equilibrium model, we propose that the dual formation strategy, which integrates interpersonal attraction and resource complementarity strategies, enables entrepreneurial teams to achieve higher entrepreneurial effectiveness by fostering a more favorable team ambidexterity trajectory slope (i.e., an ongoing increase in team ambidexterity) over time, compared to relying solely on singular strategies. We tested our hypotheses using a mixed‐method approach; this included interviews with 53 entrepreneurs from 36 real‐world entrepreneurial teams and a longitudinal study of 274 entrepreneurial teams participating in business simulation contests. Both the qualitative and quantitative data provide support for our model. These findings offer several theoretical and practical implications and suggest promising directions for future research.\n"]