Many claims have been made regarding the application of mindfulness meditation to the improvement of critical thinking skills, with some suggesting improved executive function as a mechanism. This study tests theoretical assumptions related to these claims. Sixty-five Irish university students took part in an active-controlled mixed factorial experiment designed to investigate the effects of a guided mindfulness meditation on the primary measures of executive function and critical thinking. The secondary measures assessed key thinking dispositions, including the need for cognition and actively open-minded thinking, state mindfulness and dispositional mindfulness. The 2 × 2 mixed analyses of variance showed no evidence of an effect of the interaction between time (pre vs. post) and group (mindfulness vs. sham meditation) on executive function indices (p < 0.39) or critical thinking performance (p = 0.11). No evidence was found for indirect effects of group allocation on critical thinking through either state mindfulness or executive function. Moderation models demonstrated evidence that the effects of the mindfulness meditation on critical thinking were conditional on need for cognition (b = −0.24 [−0.40, −0.08]) and actively open-minded thinking (b = −0.14 [−0.25, −0.04]) dispositions. In addition, participants who reported low levels of non-reactivity demonstrated decreased critical thinking performance following the mindfulness meditation, which was mediated by slower reaction times on the executive functioning task (b = −53.37 [−92.65, −14.08]). In summary, a brief guided mindfulness meditation appears to facilitate critical thinking for those low in need for cognition and actively open-minded thinking. However, it is unclear whether executive function is a mechanism underlying this relationship.