Health Management Information Systems (HMIS) have been used by many countries using platforms adopted or own-designed. It is implemented by integrating all health care services and enables service providers collect, store and use health data. The System provides information on all aspects of the organization from billing to patient care and beyond. Despite this significance, lack of specificity and clarity hamper understanding, implementation, functionality and evaluation. The study is aimed at assessing the functional factors for Health Management Information System performance in Elgeiyo-Marakwet County (EMC). The specific objectives were to: determine technical factors for HMIS performance in Elgeiyo Marakwet County; identify the behavioural factors for HMIS performance in EMC; determine the organizational factors for HMIS performance in EMC; determine health policy framework for HMIS performance in EMC; and identify challenges affecting the use of HMIS in EMC. Quantitative research method was used and data gathered through administration of questionnaires to 52 respondents. Data was analyzed using Statistical Package for the Social Sciences (SPSS) computer software version 20.0. The study found 85.0% of the respondents indicating availability of technical factors, behavioural factors, organizational factors and health policy framework for HMIS performance in Elgeiyo Marakwet County. In addition 87.8% of the respondents indicate that despite the presence of technical factors, behavioural factors, organizational factors and health policy framework there are also are challenges of HMIS involving factors that affect the ability to collect health data and use of health information in the county. The study concludes that the presence of standard indicators, manuals, reporting forms are essential inputs for HMIS performance in the county. In addition competency in HMIS tasks, documentation reports in the HMIS of private & NGOS and availability of minimum package of information (MPI) for collection, collation, analysis, dissemination and use of health information in HMIS are important conveyance for HMIS performance. The study recommends enhance support for technical and organizational attributes for efficient and effective utilization of HMIS, regular review as well as capacity building to alleviate behavioural attributes and challenges in the county. There is need for further study in sphere of health management information systems in the counties of Kenya.