The objective was to examine pandemic-related changes in depression and anxiety symptoms in adolescents and young adults in Germany considering pre-existing depression and anxiety problems. In this cross-sectional study, 11,523 adolescents and young adults aged 14-21 years who perceived an impact of the Coronavirus disease (COVID-19) pandemic on their mental health reported the frequencies of depression and anxiety symptoms retrospectively for different pre-pandemic and pandemic phases. Data were collected using web-based questionnaires between January 5th and February 20th, 2022. Depression and anxiety were assessed with a modified version of the Patient Health Questionnaire (PHQ-4). Scale-fit cut-offs were used to identify pre-existing elevated depression and anxiety scores. Multilevel mixed linear models were conducted to assess changes in depression and anxiety symptoms from 2019 to 2021 and compare for age, gender and pre-pandemic mental health problems. Among young people who were experiencing mental health changes as a result of the pandemic, the frequency of depression and anxiety symptoms increased during the COVID-19 pandemic. This association was moderated by age, gender, and pre-existing elevated depression/anxiety scores. For young people without elevated pre-pandemic depression/anxiety, the scores increased strongly over time, with 61% reporting elevated depression symptoms and 44% reporting elevated anxiety symptoms in 2021. In contrast, self-perceived change was minimal for adolescents and young adults with elevated pre-pandemic depression and anxiety. Among young people whose mental health has been affected by the COVID-19 pandemic, the group without pre-pandemic mental health conditions reported greater deterioration than those with elevated pre-pandemic depression and anxiety scores. Thus, adolescents and young adults without pre-existing depression and anxiety problems who perceived a change in general mental health due to the pandemic reported an alarming increase in symptoms of depression and anxiety during the COVID-19 pandemic period.