Textbooks are the only available source/s of teaching and learning English in Pakistani public education system. Whether the textbooks serve their intended purposes as mentioned in the National Curriculum of English, the researchers conducted a mixed methods teachers'-led evaluation to investigate the effectiveness of English textbooks. The study aimed to understand how Pakistani government primary school teachers in the province of Balochistan view the English textbooks of Grades 1-5 and what impacts the effectiveness of the English textbooks. Using an exploratory sequential design, the researchers first recruited 188 government teachers for survey in quantitative part of the study and later selected 12 participants from those who completed the survey for semi-structured interviews in qualitative part of the study. The quantitative findings revealed that the teachers were moderately in favor of the effectiveness of the English language textbooks of Grades 1-5. However, qualitative results revealed some direct issues such as the teachers' disagreement with the components of vocabulary and grammar in the textbooks and indirect issues such as limited time allotted for teaching English daily, teachers' lack of proper training for teaching English through the textbooks, dearth of teaching resources in the schools, and meager familial support available to students at their homes that halted the efficacious teaching of the textbooks in the Balochistan government primary schools. Recommendations have been drawn for the curriculum designers, English textbooks writers, and the department of education of Balochistan, Pakistan.