This study investigated on factors influencing mastery of reading, writing and arithmetic (3Rs) among learners of primary schools in Ilala District using descriptive and inferential statistics. Data was collected from 231 teachers in 13 sampled schools through questionnaires. The questionnaire was subjected to pilot testing, where Cronbach’s alpha results ranged between 0.760 and 0.923. The study established that school administrative support, teaching methodologies and teacher and learner related factors significantly influence pupils’ mastery of 3Rs. Pupils are perceived by their teachers to master 3Rs and school administration is concerned with pupils’ mastery of 3Rs by setting effective strategies. Pupils are perceived by teachers to attend schools regularly but they don’t have independent reading, writing habits and self discipline toward learning. Based on these findings, the study recommended that school administration should continue to support teachers’ efforts to improve pupils’ mastery of 3Rs. School administration should ensure conducive environment for learning, support reading culture, provide seminars on how to teach 3Rs and seek for support from the government and non-government organizations. Finally, teachers should encourage pupils to engage in independent reading, writing habits and self discipline toward learning.