A growing number of FOI laws have been passed worldwide, but their full enactment remains a challenge. Despite significant advances, the antecedents, conditions and dynamics driving transparency institutionalization remain largely unknown. This article aims to fill an important gap in the literature by empirically examining the unfolding of institutionalization processes in Spanish municipalities under the 2013 Spanish Transparency Law. This law imposes new legal requirements but does not mandate formal structural changes, offering a compelling case to observe organic municipal responses to legal mandates. Using a sequential mixed method strategy, we first surveyed the compliance across 1,031 municipalities. In a second step, we administered a questionnaire to transparency officers to explore their implementation strategies. Finally, we conducted 51 interviews with technicians in-charge of transparency and 14 with provincial entities that provide municipal assistance. Our contributions are twofold: first, we identify the sources of transparency implementation and analyze how institutionalization unfolds over time, highlighting the factors and conditions that increase the likelihood of success, paving new avenues for further empirical research. More broadly, our article contributes to existing debates on institutional implementation and strength.