Internet addiction (IA) by children and adolescents is a concern for parents. The intensity of this problem has increased in the context of COVID-19 pandemic across the world. Here we aimed to evaluate internet usage patterns, addiction to internet use, and mental health among Bangladeshi school-going adolescents during the COVID-19 pandemic. We conducted this cross-sectional study among 502 school-going adolescents. Pre-structured questionnaire was used to collect information related to demographics and the internet usage pattern. We assessed the prevalence of IA and loneliness using the internet addiction test (IAT) scale and UCLA-3 loneliness scale. The prevalence of IA and loneliness among Bangladeshi school-going adolescents were 88.25% and 72.51%, respectively. Individuals with English-medium education, higher classes, high economic status, mobile internet connection, online gaming habits, and living without family showed significantly higher levels of IA. Moreover, a high proportion of loneliness was observed among individuals with high financial conditions, mobile internet connection, and who watch movies on the internet. The present study findings suggest a strong association between demographics, internet usage patterns, IA, and the mental health of adolescents. These results would have practical inferences in clinical psychology, psychotherapy, and related fields. Based on this finding, the healthcare authorities and professionals can develop an inclusive interventional approach for adolescents who suffer from IA and mental health disorders.