This study explores the role of cognitive task complexity in English as a Foreign Language (EFL) curriculum development and its impact on learner engagement, motivation, and learning outcomes. A mixed-methods approach, combining qualitative and quantitative methods, was employed to gain comprehensive insights into the topic. The qualitative component involved a case study at a selected language institution, including semi-structured interviews with curriculum designers, teachers, and learners, thematic analysis of curricular documents, and non-participant observations of EFL classes. The findings from these methods provided an in-depth understanding of cognitive task complexity integration, its challenges and benefits, and potential areas for improvement. In the quantitative component, an experimental design was utilized to examine the effects of cognitive task complexity on learner engagement, motivation, and learning outcomes. Participants were randomly assigned to either an experimental group, receiving instruction incorporating cognitively challenging tasks, or a control group, receiving instruction without a specific focus on cognitive task complexity. Learner engagement, motivation, and learning outcomes were assessed through pre- and post-intervention measures, including standardized language assessments such as IELTS, and classroom performance indicators such as course grades and teacher ratings. The study concludes that cognitive task complexity plays a crucial role in EFL curriculum development and can promote learner engagement, motivation, and language development when appropriately integrated and supported. Recommendations for educational practice, curriculum development, and future research are provided based on the study's findings.