This study examined the effect of mastery learning on senior secondary school students' achievement and retention in circle geometry. The study was conducted in Ilorin, Kwara state, Nigeria. It adopted a quasi-experimental, non-randomized, pre-test, post-test control group design. A sample of 172 senior school II students was drawn from four co-educational schools using multi-stage sampling technique. Instrument for data collection was Circle Geometry Achievement Test (CGAT) which was validated by experts and reliability index of 0.82 was obtained using test-retest method. The result showed that senior school students' achievement in Geometry improved significantly when taught circle geometry using mastery learning approach. There was no gender difference found as well as no difference in the achievement of low, medium and high scoring students when taught with mastery learning approach. There was also a significance difference in the post test mean score and retention score of students taught circle geometry using mastery learning approach. It was recommended among others that trainings should be given to mathematics teachers on how to effectively use mastery learning approach in mathematics classroom if better performance is desired.