Background Since the first months of 2020, Italy and the world have been facing the COVID-19 pandemic. In addition to the dangerous and potentially deadly effects on physical health, it has caused a radical change in the lifestyle of the population and a potential danger for mental health too. These events were inserted into the context of a growing epidemiological trend regarding children’s psychiatric disorders in the past decade. Aim To study the population of patients admitted to a Neuropsychiatric Hospital Unit of North Italy in the first COVID-19 year, comparing them with the population of patients hospitalised during the year immediately before, according to sociodemographic and clinical variables. Methodology The study is an observational retrospective cohort. In total, 198 patients hospitalised due to neuropsychiatric problems from February 2019 to March 2021 were recruited. Data were analysed through mean and standard deviation, t-test, percentages, chi square test, and the Fischer exact test. Results Risk factors associated with mental health disorders were similar between the two years. The hospitalisation modality showed a decrease in scheduled hospitalisations compared to urgent ones, and among the reasons that led patients to hospitalisation there was a conspicuous increase in eating disorders. More suicidal and self-harming behaviours occurred in the COVID-19 group too, compared to the previous year. The methods used to attempt suicide were changed considerably, with a prevalence of that attempted within the home. Changes in pharmacological therapies also occurred, necessary for more than 80% of inpatients during the COVID year, with a greater use of neuroleptics. There were alarming data about hospitalisation relapses, which increased from 12.2% in the pre-COVID year to 35.0% in the COVID year. Conclusion Data shed light on clinical and policy issues in mental health care during the developmental age. Since the COVID-19 health emergency is not yet over, and its effects, especially on mental health, will be long-term, it is necessary to implement services and activities dedicated to both primary and secondary prevention of neuropsychiatric diseases especially during adolescent ages.