Although the NGOs adopt different strategic management practices to be competitive and sustainable, they still face performance challenges. This study examined the influence of strategic planning on performance of Health and Environmental NGOs in Kisii County, Kenya. The study was anchored on Stakeholder Theory supported by the Resource-Based View. Taking a mixed methods research design targeting 120 managers from 40 Health and Environmental NGOs, quantitative data was collected from a simple random sample of 102 NGO staff using structured questionnaires. A pilot study was done involving 10 Health and Environmental NGOs in Nyamira county. Data collection tools were administered through drop-and-pick and online methods. SPSS Version 25 was used to perform a descriptive, linear regression and Pearson’s correlation analyses to establish the relationships between the variables. The Qualitative data was subjected to manual content thematic analysis as secondary data from published annual reports was used to validate the primary results. The results show that strategic planning positively and significantly correlates with NGO performance. The findings inform academic, policy and strategic management practices in the NGO sector as the study recommends policies and procedures for identifying and addressing some of the concerns in strategic planning by building on the strengths noted so far to improve performance. Local NGOs should adopt the best management practices that will position the organizations for greater engagements with development partners. Regenerative activities and intensified resource mobilization are recommended to expand the financial resource base and increase NGO sustainability. For future research, this study should be replicated in other counties and further focusing on Agricultural NGOs that provide agribusiness interventions. Comparative studies should also be conducted to unearth some differences or similarities in strategic management practices and performance of the Health and Environmental NGOs in the County.