Public participation plays a significant role in the success of devolved governments in Kenya. The study therefore sought investigate the influence of public participation in county annual development plan formulation in Nyandarua county with the objectives being to determine the participants’ selection criteria during the formulation of the County annual development plan, to determine the methods of participants engagement in formulating the annual development plan in Nyandarua County, and to establish participants feedback mechanisms in county annual development plan formulation in Nyandarua County. The study was informed by two theories that are vital in unwrapping the puzzle surrounding the public involvement and engagement and are the Theory of Empowerment and the Cognitive Engagement Theory. From a reachable population of 334,932 citizens in the county, a sample size of 384 participants were selected through a snowball sampling technique. A survey research design was adopted where questionnaires and structured and unstructured interviews were utilized to gather information from the selected participants. Data analysis was done using the SPSS software (version 26.0). The results provided statistical evidence that a positive and significant influence exists between public participation and annual development plan formulation. Specifically, the three drivers; participants’ selection criteria, methods of participants engagement, and participants feedback mechanisms tested in this study were found to be significant and to have positive influence on the annual development plan formulation in Nyandarua County. The study recommends initiation of strong measures that promote public awareness in integrated development planning, Annual Development Plan program should be tailored to encompass the dynamic nature in demographic characteristics of citizens and that the county government needs to improve on public relation, reporting as well as policy communication as this was found to be positively related to public participation levels in annual development plan.