Background: Research has indicated that young learners are capable of learning mathematics because they are born with an innate core of mathematics knowledge. Teachers of young learners are, therefore, expected to offer mathematical curriculum that exposes learners to a deep and explicit knowledge of high mathematics. Aim: The study aimed at Grade R in-service teachers' understanding of teaching mathematics in their classrooms. Setting: The study sampled five in-service Grade R teachers from four districts in Lesotho, while they were enrolled in an in-service programme at a college of Education. Methods: This is a qualitative approach, and a case study design was employed. Data sources included teachers' interviews, classroom observations and document analysis for instance, the teachers' files, lesson plans for Grade R curriculum for mathematics and course outline of mathematics offered to in-service teachers during their training in the Lesotho College of Education (LCE). Which focussed on four domains of knowledge, namely, common content knowledge, special content knowledge, knowledge of content and students, and knowledge of content and teaching. Results: The findings revealed that the in-service teachers in the LCE had insufficient understanding of the teaching of mathematics, which in turn had a negative influence on the teaching of mathematics in Grade R classes. Conclusion: Despite the Lesotho government's commitment to improving the learning of mathematics at the Grade R level. Teachers' difficulties raise concerns about the effectiveness of their teaching of mathematics.