Drawing on a realist perspective (e.g., Archer, 2012; Joseph, 2002; Layder, 1997), this chapter investigates whether, why and how agency was exerted by a policymaker when designing a language-in-education policy (LEP). The agency of ‘implementers’ of language-in-education policies, such as teachers and students, has often been investigated in the literature on LEP and policy processes in general. However, the agency or appropriation of structural realities exhibited by those entrusted with decision-making powers has been under-researched. This chapter aims to develop a more comprehensive understanding of the agency of LEP actors, by providing insights into agentive acts deployed by this neglected group. It systematically analyzes the policymaking structure that sets the background for their agentive acts, and how these acts utilize, resist or detour structure. Drawing on three in-depth interviews and over 20 email exchanges with a government officer who transformed the Teaching English in English (TEE) policy in South Korea into an in-service teacher-certification scheme―first introduced in 2009 in Seoul and then expanded to the entire nation―the chapter sheds light on how an individual policy actor with power and influence maneuvered through different forces and tensions relating to the scheme at different levels and within different spaces, thus revealing valuable insights into how LEP actors negotiate structural realities. Thematic content analysis was conducted, drawing on the agency framework suggested by Block (2012). This framework includes four dimensions through which to understand the agentive language practices of multilingual individuals in a particular context: time/history, culture, physical space and semiotic resource use. The framework was slightly revised to suit the current study. Of the suggested dimensions, the fourth, semiotic resource use, has been reformulated as the policymaker’s self-reported positioning of English as a semiotic resource. By systematically analyzing the agentive acts of the policy actor in this study, and by exploring theoretical and practical implications for policy formulation, it is hoped that the current chapter will contribute to a fuller understanding of agency in LPP.