This article presents the findings from the United Kingdom's (UK's) first in‐depth exploration of the impacts of Covid‐19 on children at each stage of the youth justice system. Based on interviews with 140 youth justice professionals, participatory research with 40 children in custody and in the community, and a survey of all 157 youth offending teams in England and Wales, the research demonstrates that the pandemic increased the vulnerabilities of justice‐involved children; children who are routinely exposed to health anxieties, instability and inequalities, adverse experiences in the home, systemic racism and school exclusion. Professionals and children reported an increase in mental health illnesses such as anxiety and depression due to range of intersecting factors such as isolation, lack of socialising, lack of routine, lack of physical activity and poor sleep patterns. Post‐pandemic, there is an urgent need for a systemic commitment to addressing the mental health vulnerabilities of justice‐involved children.