Background The theory of gamified learning provides a theoretical framework to test the impact of gamification efforts upon learner behaviors and attitudes, as well as the effect of these behavioral and attitudinal changes on learning. It does so by providing mediating and moderating processes linking specific game elements to learning outcomes. Aim This article links specific game elements common to leaderboards (conflict/challenge, rules/goals, and assessment) with a focal learner behavior, time-on-task, by exploring educational research on competition and psychological research on goal-setting theory. Method The mediating process of the theory of gamified learning is tested experimentally by assigning learners completing an online wiki-based project to a gamified version with a leaderboard or to a control version without a leaderboard. Leaderboard achievement was not tied to course grades. Results Random assignment to leaderboards supported a causal effect. Students with leaderboards interacted with their project 29.61 more times, on average, than those in a control condition. Bootstrapping was used to support the mediation of the effect of gamification on academic achievement by this amount of time. Conclusion The mediating process of the theory of gamified instruction is supported. Leaderboards can be used to improve course performance under certain circumstances.