This study concerns foreign language learners' motivational changes over a long period of time; it is an investigation of the learning histories of six learners who have achieved high proficiency in English. Unlike a large body of conventional foreign language learning motivational research, which has primarily been conducted using quantitative methodologies, this study employs two non-conventional approaches, a combination of learners' biographies and case study research. The primary purpose of the study is to holistically explore successful English learners' motivational trajectories and their learning histories in the Japanese context. To this end, foreign language learning motivation is conceptualized and illustrated as a dynamically changing construct that plays an important role in the process of foreign language learning. In the literature review, longitudinal studies concerning foreign language learning motivation and autobiographical studies and case studies that are relevant to this study are examined. The central research question is what motivational trajectories and learning histories these highly proficient learners have had, and how these learners have sustained their learning motivation over time and eventually achieved high proficiency while in an EFL (English as a foreign language) environment. The participants are six Japanese adults who have achieved high levels of English proficiency and who use English in their jobs. The design used in this case study involves both holistic and specifically focused analyses, by which each participant's learning history is collected through individual interviews. The author reports each participant's learning history, and the initial proposition concerning motivational change and salient motivational sources found in the participants' learning histories are collectively analyzed and discussed. Exploring the data concerning how the participants have maintained foreign language learning motivation resulting in the idea that sustained motivation is not always present in successful foreign language learning and that the key to success involves a cognitive change from a state in which motivation is present to one in which a more intentional psychological force, commitment to learning, develops. Based on this thought, a model illustrating the key to success in foreign language learning in the EFL context is presented. The results provide new, engaging, and important information to people who are seriously involved in foreign language learning in EFL contexts, where the majority of learners fail to attain high levels of foreign language proficiency after receiving years of formal education.