The COVID-19 pandemic had an immediate negative impact on young people worldwide. However, there is a paucity of research examining the factors associated with good mental health, and specifically the factors that contributed to longer-term positive outcomes. This study aimed to identify the protective factors among early adolescents in the United Kingdom that were associated with better mental health outcomes (internalizing and externalizing difficulties, and well-being) during the second national lockdown, and any differences in protective factors between the first and second lockdowns. Between September and December 2020 (T1; N = 290), and March and May 2021 (T2; N = 72), 11 to 14-year-olds across North-West England completed an online survey pertaining to their experiences of lockdown, and mental health and wellbeing. Hierarchical multiple regression was used to analyse the data. Results indicated that peer support was protective across all three mental health outcomes at T2. While optimism was protective across all three outcomes at T1, it was not significant at T2. School support and community and family connection were also significant predictors at T1 only. While support from multiple different sources may have been more important in the early days of the pandemic, support from peers was key in maintaining long-term mental health.