The kind of environment in early childhood classroom enhances multiple sources of stimulation which encourage development of physical, cognitive, emotional, and social skills needed by learners to master concepts in various activity areas. The physical environment is crucial when it comes to mastery of various activity concepts specifically number activity concept. The purpose of this study was to investigate the influence of physical classroom environment on preschool learners’ mastery of number activity concepts in public pre-primary in Embakasi Sub County Nairobi County. The study was grounded on Lev Vygotsky’s social-cultural theory while concurrent triangulation design within a mixed method approach was adopted. 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 75 pre-primary teachers, 25 Centre managers, 3 Sub County Coordinators and 461 learners 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 physical classroom environment had a positive effect on mastery of number activity concepts with (n=61; r=.588; p<.05) strong positive correlation and it accounted for 33.5% (Adjusted R2=.335) of the variation in pre-primary learners’ mastery of number activity concepts in Embakasi Sub-County. With the F-ratio in the ANOVA F (1, 59) =31.201; p< .05, the study concludes that the regression model was well fit for data collection. The study recommends that the government finds a significant need to build more appealing classroom physical environment in public pre-primary centres to ensure child friendly environment which are warmly decorated and spacious, with good temperature and well painted walls that boosts good health and safety. The study advocates for age appropriate furniture which accommodates and allows all learners to sit properly.