Mindfulness-based interventions have been suggested as one way to improve employee well-being in the workplace. Despite these purported benefits, there have been few well-controlled randomized controlled trials (RCTs) evaluating mindfulness training in the workplace. Here we conducted a two-arm RCT at work among employees of a digital marketing firm comparing the efficacy of a high dose six-week mindfulness training to a low dose single-day mindfulness training for improving multiple measures of employee well-being assessed using ecological momentary assessment. High dose mindfulness training reduced both perceived and momentary stress, and buffered employees against worsened affect and decreased coping efficacy compared to low dose mindfulness training. These results provide well-controlled evidence that mindfulness training programs can reduce momentary stress at work, suggesting that more intensive mindfulness training doses (i.e., 6-weeks) may be necessary for improving workplace well-being outcomes. This RCT utilizes a novel experience sampling approach to measure the effects of a mindfulness intervention on employee well-being and considers potential dose-response effects of mindfulness training at work.