This study examined ways in which high school students used mobile devices in physics classrooms and after school, and the impact of in-class and after-school mobile technology use on their physics learning performance and interest. We collected data from 803 high school freshmen in China after they had used mobile devices for over five months. A fixed-effects model was employed to control the undetected variances. Results indicated that the students frequently used their mobile devices for physics learning in class and after school. Students also perceived the mobile devices as very useful for their physics learning, and their perception of usefulness was positively correlated with their frequency of use. Both the in-class and after-school duration and frequency of mobile technology use positively influenced students' physics learning achievement and interest. However, we also found that mobile technology was mainly used to augment rather than to transform instructional practices in physics classrooms. The paper concludes with a discussion of implications and limitations of these results for research and practice.