Participant adherence to standardized 8-week mindfulness-based interventions (MBIs) may be challenging, and adaptations from intervention protocols have been reported in mindfulness research. We conducted a scoping review to determine how women with breast or gynecological cancer adhered to standardized 8-week MBIs delivered in intervention studies. Searches were conducted for articles published till February 2020 in PubMed, Embase, CINAHL EBSCO, PsycINFO Ovid SP, and Cochrane Library Wiley. The following outcomes were investigated: class and silent retreat attendance, intervention completion rate (ICR), adherence to home practice, and reasons for dropping out from an MBI study. Among the 25 included MBI studies, mindfulness-based stress reduction was the most often delivered intervention and mostly women with stage I–III breast cancer were represented. The duration of classes varied from 1.5 to 3.5 hours. Planned home practice varied from 20 to 60 min/day, and silent retreat varied from 4.5 to 8 hours. Due to heterogeneity in the reporting of class attendance, the data could not be pooled. Six studies reported an average class attendance ranging from 5 to 8.2 classes. Overall, intervention completion rate (the proportion who completed all classes) varied from 26.3% to 100%; however, discontinuations were not systematically reported. Home practice time was reported in 20% of the studies and ranged from 17 to 24 min/day. The main reasons for dropping out from an MBI study were health-related problems, organizational challenges, travel distance, and lack of motivation/commitment. About 70% of the studies reported some data on participant adherence, revealing a relatively high overall frequency of class attendance. However, the monitoring and reporting of participant adherence should be improved in future studies to increase our knowledge on the required amount of participant engagement to improve health outcomes and facilitate the implementation of effective interventions on a larger scale.