Mining is an important anthropogenic disturbance of global biodiversity and ecological services. Threats to biodiversity from mining are increasing and expanding as global mineral demand increases. Offsite effects of mining are the ecological impacts occurring in landscapes around mines, and they are easily and often overlooked in impact evaluations because they occur outside the immediate location of the mining activities. Therefore, knowledge about offsite effects on understory diversity is still limited at the community level, especially in the boreal forest. We used the diversity of all vascular plants, (woody and herbaceous), bryophytes, and lichens to quantify and characterize the offsite effects of mining on the understory in a 1-km area around six mine sites in the Canadian boreal forest. The results indicated that the offsite effect of mining on understory diversity and composition was substantial, and was driven by mining stage, forest type, and distance from mine sites. The presence and magnitude of offsite effects are affected by mining stage. Generally, more negative effects on understory diversity occurred at operating mine than non-operating and control sites. Forest type is also a significant driver of offsite effects of mining. Understory species of deciduous and mixed forest were more disturbed than coniferous forests. Suitable indicators are important to be able to detect and evaluate offsite effects. Woody plants were more tolerant in the response to offsite effects than herbaceous, lichens, and bryophytes. Bryophyte cover was a good indicator of offsite effects of mining and, based on this indicator, footprint was established at approximately 0.1 km away from mine sites. Given the high stability of conifers in maintaining plant diversity and composition, a buffer zone with conifers as dominate trees around operating mine sites is proposed to offset potential adverse offsite effects on understory diversity in boreal forests.