Although past research has found that professional isolation can affect discernible work-related outcomes (e.g. job performance and turnover) and important job attitudes, researchers have not examined its impact on those less discernible but still costly work behaviours. Drawing on self-regulation theories, this study examined the effect of professional isolation on employees’ cyberloafing and time theft through self-control capacity impairment. With longitudinal data collected from 343 U.S. employees across five consecutive weeks at the early stage of the pandemic (i.e. from mid-March to late April 2020), our results of latent change score modelling analyses found that professional isolation change was positively related with changes in cyberloafing and time theft via change in self-control capacity impairment. The results increase our understanding of the hidden performance cost of professional isolation. This research also shifts the research focus from a static, between-person perspective to dynamic, within-person changes in professional isolation and related outcomes. The findings shed light on the self-regulation perspective in understanding the harmful consequences of professional isolation. Implications for future research are discussed along with practical implications for organisations.