Learner autonomy has caught the attention of educators and researchers as one of the educational goals of the 21st century and as a factor that can affect second language learning. Efforts to foster learner autonomy in English classrooms have been ongoing. The study aimed to enhance learner autonomy of lower secondary school students. Thirty-nine Thai students voluntarily signed up for the course. The development of their autonomy was examined using three instruments. First, a questionnaire adapted from Murase (2015) was employed before and after the instruction. During the course, the students were asked to keep learning logs. Lastly, based on the questionnaire results, six students were selected to be interviewed after the instruction. The data from the three sources were analyzed and used to triangulate with one another. The project-based English instruction showed positive effects on learner autonomy. The questionnaire showed an increase in students' level of autonomy overall and in each dimension: technical, psychological, political, and sociocultural. The learning logs and interviews also showed signs of learning independence over time. These findings confirm that learner autonomy can and should be fostered in English classrooms.