In our increasingly service-based world, employees are now, more than ever before, required to manage the emotional demands inherent to client interactions. These emotional demands can be fuelled by emotional display rules that are part of an organisational policy. However, what differentiates client interactions from other circumstances is that not only emotional performance standards should be met but also concurrent cognitive performance standards. In some professions, lives may even depend on the interplay between both kinds of performance. This systematic review is the first to offer a systematic synthesis of the surprisingly limited number of studies on this emotion–cognition relationship (N = 18). This synthesis clearly demonstrates that cognitive performance reduces when individuals are instructed to also modify their emotional expressions (expression focused emotion regulation) concurrently. However, although combinations of emotional and cognitive requirements most likely occur during professional events, only two studies used service simulations and none used real client interactions. Other outcomes of the systematic synthesis make it even more astonishing that the cognitive-emotional performance relationship has escaped the notice of the professional field. The most striking outcome is that emotion regulation is not only getting in the way of parallel but also of subsequent cognitive tasks.