This study assessed the implications of the African Union Mission in Somalia (AMISOM) on Kenya's national security within the context of AMISOM's transition to the African Union Transition Mission in Somalia (ATMIS). The objective of the study was analyse achievement of African Union Mission in Somalia mandates, to examine the effects to Kenya's National security and assess transition strategies of national security. The study was guided by Functionalist, Strategic and Securitisation theories. The research employed a mixed-methods research design using questionnaire surveys and interviews with respondents in Kenya's border counties most affected by Somali instability. Study findings established that, while a majority (52%) believe AMISOM has achieved its mandate to a moderate extent, concerns remain regarding the capacity of Somali Security Forces (SSF) and the overall stability of Somalia. In addition, study findings established that while progress has been made in degrading Al-Shabaab and supporting the Somali Federal Government (FGS). However, only a third of respondents believe the SSF has been adequately mentored by AMISOM. Despite success in facilitating humanitarian assistance and Civil-Military Cooperation (CIMIC) projects, nearly two-thirds believe Somalia lacks the conditions necessary for long-term stability. The study concludes that AMISOM's transition strategy inadequately addresses governance and security aspects essential for Kenyan security. Weak SSF and FGS capacity pose a threat of failed stabilization after AMISOM's withdrawal. Additionally, Kenya's current border security measures are insufficient to manage potential spill over effects. ANOVA indicated an F statistic of 134.318 and a p value of 0.000<0.05 thus the Somalia PSO strategies significantly predict Kenya’s National Security. Regressions weight model (β = -0.451, P = .000<.05) implied that Nature of Somalia PSO strategies negatively influences Kenya’s National Security. The study recommends a revision of the AMISOM transition strategy to include capacity building for FGS and SSF, including forces in federal member states like Jubaland; allocation of sufficient time for a well-monitored transition with clear benchmarks; an exploration cautious negotiations with Al-Shabaab, contingent on a strong FGS and a hybrid AU-UN peacekeeping mission to succeed ATMIS to ensure Kenya's long-term security.