The COVID-19 pandemic had varied but significant effects on the lives of adolescents. This study aimed to examine the effects of extraversion and neuroticism on changes in loneliness and negative affect among adolescents during the pandemic. Longitudinal data were collected in three waves from 673 German adolescents and young adults (Mage= 16.8 years, SDage= 0.91; female = 59%), affected by local lockdowns. The data collection was one time before (T1) and two times during the pandemic (T2, T3). Change score models were used to assess the relationship between loneliness and negative affect with consideration of extraversion and neuroticism. Results showed that pre-pandemic loneliness was predictive of changes in negative affect during the pandemic, with higher loneliness predicting increases in negative affect. Negative affect did not predict later loneliness. Extraverts showed an increase in negative affect over time, particularly between pre-pandemic measurement and the first phase of the pandemic. Higher neuroticism appeared to have increased vulnerability for negative affect during the pandemic, as a rise in negative affect were found among these adolescents throughout the course of the pandemic. In conclusion, the study highlights the significant impact of the COVID-19 pandemic on the mental health of adolescents and suggests that managing the pandemic during this specific developmental period is a challenge.