Through extensive reading (ER), EFL students can acquire language skills while becoming more motivated to read various self-selected texts. However, a challenge still exists in assessing their retention of key information while preserving their reading enjoyment. This study explores how mind mapping can be integrated into teacher guidance as an assessment tool to evaluate students’ retention in ER within a Content and Language Integrated Learning (CLIL) program. Employing an exploratory research design, the study investigates the experiences and perceptions of sixty Japanese undergraduate students engaged in ER activities. The research methodology included student-generated mind maps and a post-reading survey as well as qualitative findings derived from the students’ feedback and teacher observations. Research data demonstrated that facilitated mind mapping training could help the students to visually organize key ideas, which enables them to arrange essential concepts along with summarizing texts while enhancing their ability to remember what they read.