Behavioural responses to identical stimuli differ depending on the context, history and goals of the individual. One way to dynamically modulate sensorimotor transformations is to emphasise or de-emphasise particular spatial locations, a process called selective spatial attention. Spatial processing and goal-directed behaviour are thought to be critically dependent on the superior colliculus (SC). Evidence for this comes from local single-unit recordings that reveal a spatial organisation of visual and behavioural representations. However, the broader population dynamics and mechanisms orchestrating the dynamic direction of spatial attentional processes remain elusive. To address this, we combine large-scale and longitudinal 2-photon calcium imaging of the SC during a visual perception task to reliably assess shifts of spatial attention in mice. We find that already the retino-recipient superficial layers of the SC are involved in the enhancement of responses to rewarded visual objects. Notably, these response modulations are absent on “miss” trials or probe trials in which attention is distracted away from the relevant area, suggesting modulation by attentional processes rather than permanent changes in visual processing. Importantly, these effects are not correlated with observable behaviour, but are related to salient and robust, stimulus-independent population dynamics of the SC. Pre- and peri-stimulus population activity are good trial performance predictors, suggesting the presence of endogenous attentional dynamics. Our results shed light on a link between intrinsic neural dynamics and attentional processes in the early stages of visual processing, suggesting that spatially organised changes in SC's internal population dynamics critically influence behavioural outcomes by modulating visual sensitivity.