This study combines project-based learning with mentorship. Utilizing a sustainable development project (i.e., SEED Bank Project), this study aims to achieve multiple goals. While it attempts to promote pre-service teacher education by assigning pre-service teachers as mentors to high school students, the study critically examines the impact of a Seed Bank project to its stakeholders. The conceptual framework begins with a description of Project-Based Mentoring (PBM) and the role of Seed Bank project as a Project-Based Learning (PBL) opportunity and then examines the effectiveness of the project through the lens of PBM theory. A phenomenological case study was undertaken with nine pre-service teachers and nine high school students during the 2023-2024 academic year. The context and product evaluation areas of the CIPP (Context, Input, Process, Product) Model, along with the impact and process evaluation dimensions of Stake's Responsive Evaluation Model (REM), were selected to analyse the study's findings. The findings of the research highlight the importance of (a) the complementary use of PBL and mentorship to create a win-win context for enriched educational outcomes, (b) the significance of e-mentoring, peer mentoring, and reverse mentoring, and (c) the importance of university & K-12 collaborative projects to advance educational and sustainable development outcomes.