Measuring the construct, component processes, and practice of mindfulness remains challenging. This ecological momentary assessment (EMA) study was a pilot parallel group randomized controlled trial of a brief mindfulness practice (Brief-MP) intervention on self-reported mindfulness and performance-based cognition. Adult community smokers were randomly assigned to a Brief-MP or Control (sham meditation) group. Participants carried a personal digital assistant (PDA) for 2 weeks and were instructed to initiate 20 min of meditation (or control) training on the PDA daily, and to complete an assessment of mindfulness and cognition immediately afterwards. Additionally, participants completed assessments at random times up to four times per day. Primary outcome variables were feasibility and acceptability, state and trait mindfulness, a decentered perspective to depressive stimuli, and attentional bias to smoking stimuli. Thirty-seven participants provided EMA data totaling 1874 assessments. Fifteen of 18 Brief-MP participants completed over 75 % of mindfulness trainings in the field. The majority of Brief-MP participants endorsed favorable descriptions of the intervention: “acceptable,” and “willing to recommend to a friend”. Linear Mixed Model analyses on EMA data revealed that, over time, Brief-MP (vs. Control) increased self-reported Curiosity and Decentering assessed by the Toronto Mindfulness Scale. Brief-MP did not change trait mindfulness or cognition. In sum, Brief-MP administered in the field was acceptable and increased state mindfulness.