A growing body of research has documented the negative relationship between mindfulness and ego depletion. However, most of this research has taken a static view while ignoring the effects of mindfulness temporal changes at work. Thus, it is unclear whether the relationship between mindfulness change and ego depletion is still negative when this factor is added into the mix. To address this issue, we examine mindfulness change at the within-person level (mindfulness shift) and the between-person level (mindfulness variability) and explore their distinctive impacts on ego depletion and subsequently on counterproductive work behaviour (CWB). Drawing on ego depletion theory and mindfulness research, we propose that an upshift in mindfulness is negatively related to employee ego depletion and CWB at the within-person level, whereas high chronic mindfulness variability is positively related to employee ego depletion and CWB at the between-person level. To test our hypotheses, we used an experience sampling methodology to collect three-wave data within a day from 166 employees over 10 consecutive workdays. At the within-person level, we observed a negative and indirect effect of an upshift in daily mindfulness on daily CWB via daily ego depletion. At the between-person level, we found a positive and indirect effect of chronic mindfulness variability on chronic CWB via chronic ego depletion.