This study was carried out to examine effects of concept mapping on senior secondary students’ achievement in argumentative composition in Pankshin Local Government Area, Plateau State, Nigeria. The study utilized the quasi-experimental research design. The population of the study consisted of all the SS II students in Pankshin Local Government Area. A sample of 62 SSII students was selected and assigned to the experimental (concept mapping) and control (conventional) groups using purposive sampling technique. Four research questions and four corresponding null hypotheses guided the study. Data were collected using a test tagged Composition Writing Achievement Test (CWAT), while English Lesson Plans for Teaching Writing aided the treatment. Data collected were analysed using mean and standard deviation to answer the research questions and t-test to test the null hypotheses at 0.05 level of significance. The findings indicate that students exposed to concept mapping achieved higher in generating ideas, constructing sentences, paragraphing, and punctuation. The study concludes that concept mapping has positive effects on students’ achievement in composition writing and recommends incorporating concept mapping in teaching techniques, providing resources and support to teachers, and conducting regular assessments to evaluate effectiveness. Based on the findings of the study, it was recommended among other things that teachers of English should model concept mapping for writing to students.