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 6-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.