(1) Background: Social connectivity is key to young people's mental health. Local assets facilitate social connection, but were largely inaccessible during the pandemic. This study consequently investigates the social isolation of young adults and their use of local assets during the COVID-19 lockdowns in the UK. (2) Methods: Fifteen semi-structured Zoom interviews were undertaken with adults aged 18-24 in the UK. Recruitment took place remotely, and transcripts were coded and analysed thematically. (3) Results: Digital assets were key to young people's social connectivity, but their use was associated with stress, increased screen time and negative mental health outcomes. The lockdowns impacted social capital, with young people's key peripheral networks being lost, yet close friendships being strengthened. Finally, young people's mental health was greatly affected by the isolation, but few sought help, mostly out of a desire to not overburden the NHS. (4) Conclusions: This study highlights the extent of the impact of the pandemic isolation on young people's social capital and mental health. Post-pandemic strategies targeting mental health system strengthening, social isolation and help-seeking behaviours are recommended.