This study investigated the influence of classroom instructional resources on pre-primary learners’ mastery of number activity concepts in public preschool schools in Embakasi Sub County Nairobi County. The target population comprised of the 25 public preschools, 75 pre-school teachers, 25 centre managers, 3 Sub County coordinator and 1536 preschool learners. A sample size of 461 learners, 75 pre-primary teachers,25 Centre managers and 3 Sub County Coordinators were considered. The study utilized cluster, simple random and saturation sampling techniques to select the sample size while questionnaires, interview Schedule, and observation checklist were used to collect data. Descriptive and inferential statistics were used to analyse quantitative data with the aid of Statistical Package for Social Sciences (SPSS) version 23 while qualitative data was analysed thematically. Findings indicated that instructional resources statistically and positively affected mastery of number activity concepts in Embakasi Sub County. The study recommends that adequate instructional resources be provided in public preschools to enhance holistic development of the child. In addition, the government, teachers and parents should take the leading role in the acquisition, appropriate use and storage of instructional resources to help in improving mastery of concepts in learners.