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• Strategies generated during training predict training gains. • These self-generated strategies stabilize early on. • Externally given strategy elicits only fleeting training task gain. • Working memory training represents cognitive skill learning. Cognitive mechanisms underlying the limited transfer effects of working memory (WM) training remain poorly understood. We tested in detail the Strategy Mediation hypothesis, according to which WM training generates task-specific strategies that facilitate performance on the trained task and its untrained variants. This large-scale pre-registered randomized controlled trial (n = 258) used a 4-week adaptive WM training with a single digit n-back task. Strategy use was probed with open-ended strategy reports. We employed a Strategy training group (n = 73) receiving external strategy instruction, a Traditional training group (n = 118) practicing without strategy instruction, and Passive controls (n = 67). Both training groups showed emerging transfer to untrained n-back task variants already at intermediate test after 3 training sessions, extending to all untrained n-back task variants at posttest after 12 training sessions. The Strategy training group outperformed the Traditional training group only at the beginning of training, indicating short-lived strategy manipulation effects. Importantly, in the Traditional training group, strategy evolvement modulated the gains in the trained and untrained n-back tasks, supporting the Strategy Mediation hypothesis. Our results concur with the view of WM training as cognitive skill learning. [ABSTRACT FROM AUTHOR]