Socio-economic factors have been cited as a determinant of students’ performance in national examinations. The purpose of the study was to establish the socio-economic factors which influence academic performance in public primary schools in Murang'a South Sub County. The study adopted a descriptive survey design utilizing both quantitative and qualitative techniques. The study population entailed all the public primary schools in Murang'a South Sub County, Kenya their deputy head teachers, teachers and pupils. Due to their disproportionateness, schools were stratified according to education zones and then simple randomly sampled using the lottery technique. The study involved a sample of 21 deputy head teachers, 105 teachers and 210 pupils making a total of 336 participants. Data were collected using a pupils’ questionnaire, focus group discussions for the deputy head teachers and teachers, and an observation checklist. Data were analyzed using descriptive statistics: quantitative data were analyzed using Statistical Package for Social Sciences version 17 for windows. The socio-economic factors that influence academic performance included low parental/guardian education level; low income and pre-occupation with work. A large number of pupils could not carry out private studies at home. Due to poor economic status (low earnings) some families live in overcrowded and/or poorly lit rooms. Moreover, they could not afford to buy essential furniture, essential textbooks that were not issued at school, engage their children in extra coaching after school, buy internet enabled mobile phones and computers from which important information for education purposes can be assessed. Majority of the parents were found to be preoccupied with work. Their preoccupation denied them time to attend to school meetings/functions and observe behavior changes in their children. There is need for collaboration between the Government of the Republic of Kenya and development partners to work out, introduce and implement sustainable economic activities so as to alleviate poverty in the region. Keywords: socio-economic factors, academic performance, public primary schools, Murang'a South Sub County, Kenya