Physical activity (PA) is recognized as essential for positive physical and mental well-being in young people. However, participation in PA is known to decline as adolescents emerge into adulthood under the influence of complex social and structural factors. Globally, COVID-19 restrictions resulted in changes to PA and PA participation levels in youth populations, providing a unique opportunity for gaining insight into PA barriers and enablers in circumstances of challenge, limitation and change. This article details young people’s self-reported PA behaviours during the 4-week 2020 COVID-19 lockdown in New Zealand. Taking a strengths-based view and drawing on the COM-B (capabilities, opportunity and motivation behaviour) model for behaviour change, the study explores factors enabling young people to sustain or increase PA during lockdown. Findings are drawn from qualitative-dominant mixed-methods analyses of responses to an online questionnaire: New Zealand Youth Voices Matter (16–24 years; N = 2014). Key insights included the importance of habit and routine, time and flexibility, social connections, incidental exercise and awareness of links between PA and well-being. Of note were the positive attitudes, creativity and resiliency demonstrated as young people substituted or invented alternatives to their usual PA. PA needs to change to adapt to new circumstances over the life course, and youth understanding and knowledge of modifiable factors may provide support for this. Thus these findings have implications for sustaining PA during late adolescence and emerging adulthood, a life phase that can be associated with significant challenge and change.