The present study investigates whether task engagement mediates the effects of reading-related self-efficacy and motivation on reading comprehension. Finnish-speaking students from Grades 3 and 4 performed a computer-based reading comprehension task. Engagement was measured by two indicators retrieved from log files: time-on-task (behavioral engagement) and response strategy (cognitive engagement). Two studies were conducted. Study 1 included mostly fluent readers (N = 108), and Study 2, which served as a conceptual replication of Study 1, included both fluent and dysfluent readers (N = 308). The results of both studies suggest that cognitive engagement mediates the effect of reading enjoyment on reading comprehension. In Study 2, the effect of self-efficacy on reading comprehension was mediated by both types of engagement but only when the effect of reading enjoyment was not controlled for. Overall, the results provide further clarification of the mechanism by which motivation affects children's task-oriented reading.