This mixed-methods case study examines how an online extensive reading project impacted the 11 participants' motivation, habits, and linguistic abilities regarding L2 English reading. The participants engaged in extensive reading for one year on Xreading (an online extensive reading website). They were all female Japanese nationals aged between 30 and 65 whose L1 was Japanese. Previous studies have reported the benefits of extensive reading for elementary/intermediate learners, but have not fully looked at outcomes for learners at higher levels. This study's findings suggest that extensive reading is equally beneficial for those above the intermediate level. Throughout this project, the participants accessed graded readers on Xreading, and quickly established reading habits in English, transforming from reluctant to engaged, avid L2 readers. The qualitative data obtained from interviews, journals, and records on Xreading revealed the sources of the participants' positive changes to be: how engaging and easy to comprehend the graded readers were, a sense of commitment to the project, consistent support from the researcher, and the Xreading's useful functions. Additionally, the participants' vocabulary sizes and reading rates, increased significantly (vocabulary size gains, p < 0.0005 and reading rate gains, p < 0.05) in parallel to the substantial amount of reading they were doing.