This study delves into the nature of the United Nations Mission in South Sudan (UNMISS) and specifically explores UNMISS's key roles like violence reduction, political instability, the security situation, social cohesion, reduction of violence, protection of civilians and socio-economic development. The study was guided by two objectives; to assess the objectives and mandates of UNMISS, and to examine the effectiveness of the roles and mandates of UNMISS in addressing the underlying causes of conflict and fostering sustainable peace in South Sudan. The study was underpinned by two theories, conflict transformation and Liberalism institutionalism theory. The study adopted a mixed method design with a target of 3280 and a sample size of 354 which was determined by the Yamane (1967) formula. The study employed purposive, cluster, stratified, and systematic sampling. Primary data was collected through interview schedules and questionnaires which were supplemented by FGDs. While secondary data was collected through a review of various publications. Quantitative and qualitative data were analyzed using SSP version 28 and thematic excerpts respectively. The study concludes that UNMISS has been in the front run to de-escalate hostilities by putting in place several strategies to create a conducive environment that supports dialogue and negotiation. This includes concerted DDR efforts that aim to reduce armed conflicts and prioritize the protection of civilians by fostering trust within the warring communities. Equally, UNMISS has played critical roles in ensuring social cohesion and political stability that are conducive to reconciliation and dialogue at the grassroots level. Peace-building initiatives have been achieved through missions’ commitment to the protection of human rights, social-economic development and availability of resources. The study recommends a substantial increase of resources to UNMISS and strengthening the SSR and DDR programs.