This study was conducted to identify the level of practice of critical thinking skills (CTS) and to determine whether the dimensions of perception and readiness of the teachers were the predictors of the implementation of the CTS. A cross-sectional quantitative approach using questionnaires have been used on 226 Mathematics teachers teaching in high performing school (HPS), medium performing school (MPS) and low performing school (LPS) in Kelantan, Malaysia. Descriptive statistical analysis showed that the level of practice of CTS in HPS was higher than MPS and LPS. Post-Hoc Multiple Comparisons test also showed no significant differences for perception and teacher readiness mean scores towards CTS between HPS, MPS and LPS. Furthermore, Stepwise's multiple regression analysis shows that perception and teacher readiness is a predictor factor in the practice of applying CTS in the Mathematical learning and teaching process in schools. This study provided a significant contribution towards improving the practice of CTS in Mathematical learning and teaching to realize educational excellence across the 21st century learning that emphasizes high-order thinking skills (HOTS). Hence, teachers should plan their lessons well by selecting effective strategies and teaching and learning materials that will inculcate and enhance CTS in students simultaneously achieving the teaching and learning objectives.