This sequential, explanatory mixed-methods study examined students' intentions to stay in or leave engineering industry careers upon graduation. We gathered survey data from 128 second-year engineering students about their intentions and attitudes. The participants were enrolled in a two-week interdisciplinary engineering summer course at University College London called How to Change the World. From this survey, we also interviewed 15 students, eight intending to stay and seven intending to leave, about their intentions and experiences that informed those decisions. We found that students' perceptions of future (motivation), expectations, experiences at university, confidence in their ability to succeed in engineering courses, and sense of belonging in engineering industry were the main contributors to their decisions to stay or leave. The two data streams combined provide a richer picture of how students may be better supported during and through their engineering degree programmes.