In this article, we explored learners' conceptual understanding of coordinates and transformation geometry through concept mapping. A qualitative case study was employed in this study, and data was collected using an investigative task, observation and reflective interviews on a sample of 34 Grade 12 mathematics learners. The study was guided by the following two research questions: What are the learners' understanding of coordinate and transformation geometry? and How do concept maps improve learners' understanding of coordinate and transformation geometry? The study revealed that, although some learners struggled with linking words and omitted some concepts in their concept maps, there were some indications of conceptual understanding of coordinate and transformation geometry when the learners were probed during the interview. Thus, we argue that the learners' conceptual understanding of coordinate and transformation geometry could be improved when they are taught, using concept mapping. The learners actively situated their knowledge of small pieces of coordinates and transformation geometry; and their understanding of coordinate and transformation geometry was expanded to show the conceptual structure of representing given information and a complete picture of the situation. It is therefore recommended that the teachers allow the learners to use the concept mapping strategy to construct their knowledge and that concept maps assist the learners in focusing on main ideas of identified concepts and explain the relationship.