Do people tend to underestimate time when their attention is engaged? Studies supporting this idea have routinely confounded attentional manipulations with changes in other factors, such as response complexity and memory load. The aim of the present study was to obtain the first direct evidence that attentional engagement mediated by cortical brain mechanisms affects time estimation. Participants were asked to perform a visual search task that either should not demand attention (simple feature search) or should demand cortical attentional engagement (conjunction search). Observers searched through 2, 4, 8, 16, 24, 32, or 40 items, for blocks of 40 or 60 trials. At the conclusion of each block participants were required to provide a written estimate of block duration. This time estimate was prospective in nature because subjects knew in advance that they would be asked to produce the estimate. Results showed that an attentionally demanding conjunction search task produced a large underestimation of time. And as the engagement of attention increased so did the underestimation of time. These findings provide strong support for an attentional model of prospective time estimation that is subserved by cortical brain mechanisms.