With the COVID-19 pandemic, the Internet has become a key player in the daily lives of most people. We investigate the relationship between mental health and internet use frequency and purpose six months after the first lockdown in the UK, September 2020. Using data from the UK Household Longitudinal Study on the 12-item General Health Questionnaire and the Internet use module, and controlling for sociodemographic characteristics and personality traits, we find that older individuals (aged 59 or above) have a lower internet use frequency (twice a day or less). Younger women use the Internet for social purposes more than men do, while younger men use the Internet for leisure-and-learning purposes more than women and older men do. Both high frequency internet use and use for social purposes appear to be a protective factor for social dysfunction. Interestingly, high internet use is a protective factor for social dysfunction among younger women, but a risk factor for psychological distress among younger men. Finally, while leisure-and-learning purpose is a protective factor for social dysfunction among younger women, it is a risk factor for social dysfunction among younger men.