Portfolio assessment, as an alternative writing assessment approach, has received growing attention in the past few decades. Although the benefits of portfolio assessment are well validated, there is a dearth of empirical research on how portfolio assessment can be sustained over time and the support teachers need to sustain portfolio assessment practice in their teaching contexts. To fill this significant void, the present study examines the influences that contribute to the sustainability of portfolio assessment in second-language writing. Drawing on data from interviews with the principal, English department chair and four English teachers from one elementary school in Hong Kong, as well as classroom observation and teachers’ team meeting observation, the study revealed that administrators’ role in dispersing decision-making authority to teachers, exploiting learning opportunities and providing a stimulating environment for teachers, and the sharing of common vision and goals, as well as collective flows of learning among team members, are the cornerstone of transformation and sustainability for the practice of portfolio assessment. The paper concludes with practical implications on how the innovative attempts in portfolio assessment can be sustained over time. Copyright © 2024 The Author(s).