The use of mobile health technologies (mHealth) to ameliorate HIV care has considerably risen in low- and middle-income countries (LMICs) since 2010. Yet, the discrepancies in the results of accompanying studies warrant an updated and systematic consolidation of all available evidence. We report a systematic review of studies testing whether text/image messages, interactive voice response reminders, or calls promote adherence and retention to antiretroviral therapy (ART) in LMICs. We systematically compiled studies published in English until June 2018 from PubMed/Medline, Web of Science, WHO database, ProQuest Dissertations and Theses, and manual search. We used the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2009 and used frequency analysis to assess reported findings. In total, we compiled 35 published articles: 27 completed studies and 8 protocols. Among the main 27 studies, 17 examine adherence, 5 retention, and 5 both measures. Results indicate that 56% report positive and statistically significantly impacts of mHealth on primary outcomes, the remaining 44% report insignificant results. While 41% of studies found a positive and significant effect for adherence, only 12% improved retention. The evidence shows ambiguous results (with high variability) about the effectiveness of mobile phone-assisted mHealth interventions to boost adherence and retention to ART. The literature also points to short follow-up periods, small samples, and limited geographical coverage. Hence, future research should focus on evaluating longer interventions with more patients spread across wider areas to address whether mHealth can be effectively used in LMICs.