This mixed-methods study investigated and validated the sustainability threshold concept among multi-disciplinary engineering students, taking into account students’ perceptions and the impact of the sustainability threshold concept on student understanding of sustainability in their respective disciplines, while employing the constructivism paradigm. Sustainability education faces numerous obstacles, and under these conditions, some disciplines struggle to connect sustainability to their field. This study utilised threshold concept theory and liminality to develop a new model to help connect students’ perception, liminality, sustainability, and key competencies in sustainability education. Three years of data collection in the midst of pandemic led to the development of a questionnaire for 100 students enrolled in a multidisciplinary sustainability course. In addition, a total of 25 participants were interviewed across a variety of disciplines. Findings indicate that there is a correlation between student perceptions and learning about sustainability; more advanced students (fourthyear) viewed sustainability as less relevant than novice students (first-year). Through the development of a novel model based on the threshold concept, this study contributes to a better understanding of students’ perceptions and experiences of sustainability education. This study sheds light on both the opportunities and challenges of teaching sustainability in engineering education. It also emphasises the importance of students’ perspectives in learning about sustainability.