Contradictory findings in reviews that assess the relationship between the Mediterranean Diet (MedDiet) and cognitive functioning have been attributed to heterogeneity in the criteria used to assess MedDiet and cognition, and differences in the location or cultural habits of the populations studied. Few reviews have examined the relationship between dietary adherence and cognition or considered the impact of sex differences. This systematic review and meta-analysis examines the relationship between adherence to the MedDiet and cognitive functioning of healthy older adults (50+). We included isolated data from Mediterranean regions and considered differential outcomes based on sex. A search of PubMed, Cochrane Library, EMBASE, Scopus and Web of Science from 2011 to 2018 identified five longitudinal cohort studies (n = 3368) and one randomized controlled trial (RCT; n = 239 intervention; n = 95 controls). The primary outcome of interest was global cognitive functioning. Meta-analysis of cohort studies revealed a significant association between MedDiet adherence and cognitive functioning (n = 3368, r = 0.09, p = 0.012), an effect greater than previously reported. Meta-regression demonstrated that the effect size was stronger with improved quality of study reporting (p = 0.01). Sex did not impact cognitive outcomes, but sex differences in levels of adherence were reported in individual studies. There might be a stronger association between MedDiet and cognitive functioning for older adults from Mediterranean countries compared to other geographical locations, perhaps due to higher adherence and/or longer exposure over the life course. Results are compared to those of prior meta-analyses, and the impact of sources of heterogeneity is considered. The potential influence of sex and gender, as biological and social constructs, on dietary adherence is discussed.