Information and communication technology are promising channels in delivering the mindfulness-based intervention. A common problem in these interventions is the lack of study of treatment adherence. The current article summarizes the state of measuring and reporting adherence, the rate of adherence, and possible predictors of adherence in the form of a systematic review following the PRISMA statement. A database search of PubMed, Web of Science, Medline, Scopus, and PsychINFO identified 3104 potentially relevant articles published up to the end of December 2019. The review focuses only on the SARS-CoV-2 pre-pandemic time because the accelerated development of the eHealth Mindfulness-based interventions and a vast amount of newly published interventions in the following years was expected. A total of 69 studies met all the eligibility criteria. Out of these studies, 61% reported some measure of adherence and 36% reported the adherence rate. The adherence rate in clinical populations ranged from 41% to 92%; in non-clinical populations, it ranged from 1% to 85%. Predictors of adherence were investigated in 48% of the studies; however, the majority of assessed variables had mixed findings about predicting adherence. There was a huge variety in the definitions and measurements of adherence. A standardized system of measuring and reporting adherence and further investigation of its predictors is needed.