The worldwide spread of the coronavirus disease 2019 (COVID-19) pandemic in early 2020 affected all major sectors, including higher education. The measures to contain the spread of this deadly disease led to the closure of colleges and universities across the globe, disrupting the lives of millions of students and subjecting them to a new world of online learning. These sudden disturbances coupled with the demands of a new learning system and the experiences of living through a pandemic have placed additional strains on the mental health of university students. Research on university students' mental health, conducted during the pandemic, have found high levels of stress, anxiety and depression among students. In this qualitative study, we aimed to understand how pandemic experiences have affected student well-being by conducting in-depth interviews with 34 undergraduate students enrolled in a UK university. All interviews were conducted through Microsoft Teams and recorded with prior permission. Transcripts of recorded interviews were thematically analysed which identified two broad themes: (i) University students' mental health and well-being experiences during the pandemic; (ii) factors that influenced students' mental health and well-being. These factors were further distributed across six sub-themes: (a) isolation; (b) health and well-being; (c) bereavement; (d) academic concerns; (e) financial worries and; (f) support, coping, and resilience. Our study identifies the importance of mental health support to university students during pandemics and calls for measures to improve access to support services through these crisis points by universities. Findings can also inform students' mental health and risk assessments in the aftermath of the pandemic.