The current daily diary study among 60 dual-earner couples examined whether daily levels of mindfulness at work were associated with both the employees and their partners’ well-being. Based on the spillover-crossover model, we hypothesized that on days when the employees’ state mindfulness at work was higher, it would spill over to the home domain in the form of an increased state happiness at the end of the day and decreased work to family conflict. Furthermore, we hypothesized a crossover of mindfulness at work between the members of the couple, so that the partners of employees who were highly mindful at work would be more satisfied with their relationship. We examined all our hypotheses from a daily, within-person perspective. Participants filled in an online diary survey during five consecutive working days (N = 120 participants and N = 600 occasions). The results of the multilevel analyses showed a spillover effect from the employees’ state mindfulness at work to their state happiness and their spouses’ report of the employees’ work-family conflict. Moreover, we also found a crossover effect between mindfulness at work and spouses’ relationship satisfaction. Finally, results supported a partial mediation model in which daily mindfulness at work was positively related to the daily spouses’ relationship satisfaction and negatively to employees’ spouse-reported work-family conflict through the employees’ daily happiness levels. Therefore, these findings suggest that mindfulness at work influences not only the employee, but also affects the family domain by reducing strain at home and increasing relationship satisfaction.