Evolving trends of 21st century require the continuous development of individuals to become competent in using the skills of era effectively. Critical thinking (CT) is one of the basic skills of the century for the intellectual development of individuals to sustain global welfare. Infusion of CT into the subject matter content is one of many efforts for the training of individuals to think critically. Thus the derive for this study was the possibility of CT training in a secondary school context with elementary level of EFL learners. Having concurrent embedded mixed method research design; this study used various data collection methods. Quantitative data was gathered through a CT skills scale conducted as pretest and posttest. The structured interview was the qualitative tool to gather the opinions of the participants about the CT embedded English learning process. The results showed that the treatment group statistically outperformed the control group in the development of CT skills. Qualitative data indicated that the treatment was effective for the improvement of both language and CT skills. Moreover, the instruction process was motivating for the learners due to its meaningful, fun, authentic and supportive nature. The results have implications for EFL teachers that the full integration of CT to the English course objectives, language learning activities, assignments, assessment and teacher attitude is possible and supportive for the improvement of low-proficiency secondary school EFL learners as qualified thinkers in the target language.