Teaching is a demanding profession with teachers of very young children reporting high rates of stress and exhaustion. We tested the effects of a relationship-focused professional development intervention designed to enhance teachers' use of mindfulness-based strategies to support coping on trajectories of teachers' stress, exhaustion (emotional, physical, and mental), and coping. Infant and toddler teachers (N = 81) from Early Head Start (EHS) or EHS childcare partnerships (CCP) were randomized to the intervention or usual care control condition. Using ecological momentary assessment, teachers completed twice-weekly reports of stress, exhaustion, coping, and coping strategy effectiveness via smartphones for 40 weeks. Multilevel linear regression modeling, accounting for within-person repeated measures, showed no intervention effects on stress and exhaustion trajectories. Teachers in the intervention reported increased use of mindfulness-based strategies for coping over time as compared to the control group, although frequency of use peaked and then declined. While perceptions of stress and exhaustion did not change, teachers' increased use of mindfulness-based strategies suggests improvements in how teachers managed stress and exhaustion; however, the decline in use of coping suggests the need for ongoing support within the workplace.