The purpose of this study was to determine factors that influence the performance of Science subject in public primary schools, Westlands sub-county. The study used Porter’s et al. (2008) approach on leadership and Dzama’s (2012) approach. The approach focusses on the effect of teaching and learning materials on performance of Science, how the head teacher’s professional leadership affects performance in Science, how syllabus coverage affects pupils’ performance in Science, and to assess the pupils’ attitude towards Science. The study was guided by the constructivist theory as postulated by Jean Piaget (1934). The target population was all the 25 public primary schools in the sub-county. The sample size was 8 head teachers 40 Science teachers and 142 pupils. Questionnaire sets were used to collect the required information. Secondary data on performance was collected from the District Education office. Data was analysed by the use of the Statistical Package for Social Sciences (SPSS) version 24. Spearman rank-order correlation was used since the data was ordinal. It was established that Science was the worst performing subject over the past six years, provision of teaching and learning resources for Science directly influenced performance, School leadership positively affected performance, and syllabus coverage did not significantly contribute to the learners’ performance. Learners had a positive attitude towards Science. The study recommended that MOEST provides adequate capitation grants to schools for the procurement of teaching and learning resources for Science, particularly for the practical lessons. Further research was suggested on the appropriate methodologies for teaching Science at primary school level.