This systematic review examines the empirical literature published between 2015 and 2021 on mobile-based microlearning in adult learning contexts. The rapid shift to online learning in 2020 in response to the global COVID-19 pandemic has emphasized the need to explore flexible learning options for adult learners. The convenience of mobile-based learning has increased due to the prevalence and global access to mobile devices. Mobile-based microlearning is an emerging area of research, and in this systematic review we explore ways adult learning contexts -- including workplace and higher education -- have integrated mobile-based microlearning to support instructional goals. We synthesize nine articles about mobile-based microlearning highlighting findings and implications for facilitators. Our findings showed that mobile-based microlearning is being implemented in various instructional contexts and the included studies focused on effectiveness and design principles. We conclude our review with recommendations for implications for practice.