This article examines the changing pattern of key party position assignments in three leader-centric political parties during the democratization of South Korea. We examine the interplay of three key variables: (1) party members’ relationship to the party leader, (2) leaders’ relationships to the party’s core faction, and (3) political seniority, as factors that determine who receives key party positions. More specifically, we focus on the way seniority interacts with factionalism and patrimonialism to affect the assignment of key positions. Our analysis highlights the substantial variation between our focal parties depending on the configuration of these three variables. In contrast with a conventional view that has focused on uniformity in South Korean intraparty politics, we find that a seniority-based institution has emerged for the assignment of key party positions. Our study contributes to an understanding of the interplay of personalistic behaviors and informal rules in intraparty politics and how this interplay contributes to the institutionalization of informal but important institutions governing intraparty politics during democratic consolidation.