This paper examines mediation efforts in intrastate conflicts involving regional mediators. I hypothesize that regional mediators with higher levels of legitimacy are more likely to yield successful mediation efforts. The findings of this study only partially support this hypothesis. While a positive correlation was observed, the small-N nature of the study and certain limitations prevent a stronger statement of support for the hypothesis. A great deal of focus on the research generated in the intrastate mediation literature focuses on the effectiveness of bias versus unbiased mediators; the reasons for which they are effective, usually related to the actor’s economic and military leverage over the warring parties; and the different outcomes and conceptualizations of successful mediation efforts. Less attention, however, has been given to ideational sources for effective mediation such as the legitimacy of a mediator. Duursma’s (2020) recognition of this gap has opened a new avenue for the mediation literature to explore in the quest to strengthen the effectiveness of mediation efforts at resolving armed conflicts. This leads to the following research question: Why are regional mediators effective in some cases but not others? To answer this question, I will conduct a Structured, Focused Comparison study of the mediation efforts in two Southeast Asian nation-states, the Philippines and Thailand, facilitated by another Southeast Asian nation-state, Malaysia. Mediation efforts in one case, the Moro conflict of the Philippines, was deemed successful and was solidified via the signing of a comprehensive peace agreement, while mediation efforts in the other, the Malay Muslim conflict in Thailand, has remained unsuccessful thus far. This paper seeks to shed light on a trait that could make regional mediators uniquely effective.