The purpose of this study is to compare the proportion of the time spent on smartphones in South Korean’s leisure life from the perspective on colonization of leisure and explore its implications before and after COVID-19. In other words, analyzing data of ‘The National Leisure Activity Survey’ and ‘The Korean Media Panel Survey’ from 2019 to 2020, this study intended to examine whether the proportion and influence of smartphones in South Korean’s leisure time have increased since the outbreak of COVID-19. The key findings are as follows. Firstly, compared to the period before COVID-19, leisure time increased on both weekdays and holidays, and the time to use smart devices including smartphones during leisure time also increased. Second, it was found that the percentage of time spent using smart devices during leisure time has increased by nearly 20% since COVID-19, accounting for more than half of leisure time. In particular, teenagers accounted for 80% and those in their 20s and 30s accounted for more than 70%. Third, the number of smartphone users increased in most time windows measured in every 15 minutes for 24 hours a day except for sleep time. Fourth, when smartphones were used simultaneously with other media, the number of users increased the most from 7 p.m. to 10 p.m. called ‘Prime Time’, which is assumed to be due to the increase in the number of people watching TV and using smartphones at the same time. Lastly, Though TV still accounted for a large part of our leisure time, it is found that smartphone is the medium that controls the rhythm of our daily lives. Taken as a whole, the deepening of colonization of leisure by smartphones after COVID-19 has been verified. In this vein, we suggested that future studies should be conducted critically in consideration of the economic and technical systems related to smartphones, such as platforms and algorithms that induce excessive use of smartphones.