This study examines how individuals adapt to varying levels of prospective memory (PM) and ongoing-task difficulty in a cognitively demanding task environment. PM – the ability to remember and execute delayed intentions while engaged in other tasks – depends on flexible allocation of limited-capacity cognitive resources and strategic cognitive control. Prior research has rarely manipulated both PM and ongoing-task difficulty within the same study, has typically used relatively undemanding tasks that unlikely exceed human capacity to manage them, and has often relied on aggregate performance measures that cannot disentangle capacity sharing from cognitive control. To address these limitations, we orthogonally manipulated PM and ongoing-task difficulty within subjects using a demanding air-traffic control task and applied Prospective Memory Decision Control – a cognitive process model that jointly quantifies individuals’ capacity allocation and cognitive control strategies. As PM difficulty increased, participants reallocated cognitive capacity from the ongoing task to the PM task and adopted a cognitive control strategy that favoured PM: raising decision thresholds for ongoing responses and lowering them for PM responses. By contrast, increased ongoing-task difficulty impaired information processing quality for both tasks (i.e., reduced signal-to-noise ratio) and led participants to lower PM thresholds without changing thresholds for the ongoing task. These findings provide model-based evidence consistent with distinct contributions of capacity sharing and cognitive control within the PMDC framework and illustrate the value of formal modelling in a demanding PM setting. Key Points• Successful prospective memory (PM) requires an ability to concurrently manage PM and ongoing task demands.• We present and test a cognitive model that explains how individuals use selective attention (capacity) and cognitive control to adapt to changes in PM and ongoing-task difficulty in a demanding air traffic control task paradigm.• Individuals adapt to increased PM difficulty by reallocating (sharing) cognitive capacity from the ongoing task to the PM task, and by adopting a decision-control strategy favourable to PM responding.• Increased ongoing-task difficulty reduced the quality (signal-to-noise ratio) of information processing for both tasks (i.e., reduced/shared capacity).• Successful adaptation to PM and ongoing-task difficulty can rely upon capacity sharing and the strategic use of cognitive countermeasures.