This study aimed to examine the determinants of public sector reforms on service delivery in the Ministry of Interior and Internal Coordination, Isiolo County, Kenya. The objectives of the study were; To examine the effect of capacity building on service delivery in public sector to people of Isiolo County; to examine the effect of performance management system on service delivery in Isiolo County. The respondents were all employees working in all the Ministry of Interior and Internal and Coordination of National Government, Isiolo County. The theories which guided the study were Ability, Motivation, Opportunity Theory, Reinforcement and Equity Theory. The target population for the study comprised of 3108 permanent employees working in the ministries and departments in the County Government of Isiolo where a sample population of 354 was used. Primary data was used in this research and was collected by use of questionnaires. Three types of validity were used in this study namely; content validity, construct validity and face validity. Reliability of the research instrument was determined using Cronbach’s Alpha coefficient where a coefficient value of greater than 0.70 from a pilot study data showed that the instrument was reliable. Descriptive statistics were used for quantitative data analysis which enabled the researcher to describe distribution of scores and even measurements. The collected raw data from the field was coded and transcribed and translated into quantitative data and then analyzed using Statistical Package for Social Sciences (SPSS version 24). Correlations were used to test the strength of the relationship between the variables; independent variables and the dependent variable. The findings revealed that capacity building and performance management system significantly impacted the service delivery in the ministry of interior and coordination of national government as indicated by P-value of 0.003. The government should implement comprehensive capacity building programs that address the diverse skill sets required within the Ministry of Interior and Coordination of National Government. This includes technical skills, leadership development, and specialized training to keep the workforce abreast of emerging trends and technologies in the public sector.