Research suggests that improving executive function (EF) skills using mindfulness-based interventions is promising, though few studies investigate mindfulness-based interventions for young children. The current study evaluated preliminary data examining EF over the course of a 12-week mindfulness-based curriculum implemented within preschool classrooms. Study 1 suggested that 4-year-olds' performance on the Head-to-Toes EF task improved over the curriculum. However, behavior in the Gift Delay EF task appeared to decline from early intervention to post-intervention. In Study 2, the teachers' retrospective reflections on performance suggested that they anticipated children would improve on EF across the intervention, potentially more on the Head-to-Toes task. The teachers suggested that children peeked earlier on the Gift Delay because of testing conditions (e.g., time of year, differences between measures) and learning experience (e.g., earlier peeking without consequence). This study demonstrated mixed results for mindfulness-based interventions related to EF performance in preschoolers and the need for future research incorporating several practical considerations for understanding how behavioral testing may inform classroom interventions.