This preliminary study aimed to understand the effects of an autonomous mindfulness-based intervention (MBI) on mindlessness propensities: rumination, automatic pilot functioning, and attentional distractibility. The ecological momentary assessment was completed by community participants assigned to two nonrandomized groups: an experimental group (n = 45) that practiced 20-minute daily mindfulness meditation for 42 days and a control group (n = 44) that was on the waiting list for the MBI. All participants completed a self-assessment on rumination and mindlessness propensities twice a day. The MBI led to a favorable gradual decrease in automatic pilot functioning and attentional distractibility. Rumination evolved in three stages: a rapid decrease during the first week, a stabilization phase between the 10th and 30th days, and an additional decrease after 30 days of practice. This innovative study provides a promising perspective regarding rumination, automatic pilot functioning, and attentional distractibility dynamic trajectories over the course of an MBI.