This paper reports on a longitudinal case study of a Hong Kong early career ESL (English as a second language) secondary teacher, Joyce (pseudonym), who experienced different stages of personal–professional development over seven–eight years (August 2013–December 2020), as follows: (1) entering, and engaging, in teaching for five–six years, upon graduation from a local teacher education BA degree program in summer 2013; (2) resigning from her full-time teaching position and leaving the teaching profession, in response to an “insulting” classroom revisit in her third school; (3) working in an NGO for a short time, after “recovery” from the “insulting” event; and (4) weighing possibilities for resuming teaching, after leaving the NGO in 2019. Drawing on multiple data that were collected over seven–eight years, including interviews, informal communications, and autobiography, this study aimed to examine the issues of teacher attrition and sustainable professional development, in relation to teacher agency and teacher identity, in Hong Kong secondary school contexts. The findings revealed that school and social contexts intertwined with personal experiences, culminating in Joyce’s leaving or staying in the teaching profession. Through focusing on Joyce’s long-term experiences of becoming and being an ESL teacher, the findings shed light on the affordances for, and constraints upon, teacher agency and teacher identity in school contexts. Copyright © 2021 MDPI AG