Management by Objectives (MBO) is a performance management approach where employees and managers collaborate to set individual goals aligned with broader organizational objectives. While MBO's potential benefits are recognized, its effectiveness, particularly in the context of varying organizational cultures and appraisal purposes, remains unclear. This study aimed to investigate the relationship between MBO and employee performance within Isiolo County Government. Previous research in Kenya has primarily focused on specific MBO components like monitoring and evaluation rather than its overall impact on performance. To address this gap, the study examined the influence of planning, monitoring, and evaluation on employee performance in Isiolo County. Grounded in socio-analytic, mirror, and agency theories, this descriptive research involved 260 employees across seven county ministries. Data was collected using a questionnaire subjected to pilot testing and reliability analysis (Cronbach's alpha = 0.78). Content validity was established through expert review. Statistical analysis revealed evidence of effective planning, monitoring, and evaluation practices in Isiolo County. Process review and monitoring significantly influenced evaluation outcomes. Additionally, planning and monitoring positively impacted overall employee performance. However, the study found no significant influence of evaluation on performance. Based on these findings, the study recommends continuous improvement in planning and monitoring processes to enhance employee performance. Moreover, there is a critical need for organizations to refine their evaluation systems. To gain a deeper understanding of the long-term impact of evaluation on employee performance in Kenyan county governments, longitudinal research is warranted