Language learner engagement, which is receiving increased attention, has predominantly focused on offline classroom contexts, while learner engagement in language Massive Open Online Courses (LMOOCs) remains under-explored. This study was conducted on a College English MOOC with the purpose of examining learner engagement and its relations with autonomy and academic performance in the LMOOC. In the first phase, the learning analytics (LA) approach was adopted. This involved collecting tracking data of 3673 learners from the learning management system (LMS) of the LMOOC platform. The data included three types of activities (videos watched, assignments submitted, and posts written) as indicators of learners' online task engagement and scores on the final examination as a measure of their academic performance. In the second phase, 115 learners of the focal LMOOC responded to a survey that measured autonomy and three dimensions of engagement (behavioral, cognitive, and emotional engagement) typically explored in second language (L2) learning research. The results of the LA approach showed that all three indicators of online task engagement significantly predicted academic performance. The survey results indicated that only cognitive engagement among the three dimensions of engagement significantly predicted academic performance. While autonomy did not predict academic performance, cognitive engagement significantly mediated the relationship between autonomy and academic performance. Implications for LMOOC instructors, designers, and researchers are then addressed.