The beneficial effects of the Mediterranean diet (MD) adherence in reducing cardiovascular disease (CVD) risk and improving CVD-related physiological indices have been well-documented. However, the exact MD adherence duration needed for these effects to occur is under-researched. The aim of the present, two-arm, two-site study clinical trial was to assess the effects of long- vs. short-term MD adherence on the skin microvascular circulation, and quality of life. Two groups were recruited, one being long-term MD adherers (>5 years; from Greece; control group), and one of the non-adherers (from the UK), with the latter participating in a four-week MD intervention (intervention group). Our main outcome was skin microvascular function assessed by cutaneous vascular conductance (CVC). Secondary outcomes included quality of life, dietary intake, blood pressure and lipidemic profile. At the end of the intervention, both groups had high MD adherence. For the intervention group, significantly improved post-intervention CVC values were noted concerning the initial peak phase (2.0 ± 0.6 vs. 2.8 ± 0.8; < 0.05). CVC values of the control group, were however higher at the plateau phase in comparison to the intervention group (intervention end; 3.8 ± 0.8 vs. 3.1 ± 1.2; < 0.05). As per QoL, the physical domain was improved post-intervention (13.7 ± 1.2 vs. 15.9 ± 1.2; < 0.05). No differences were observed in the lipidemic profile between groups, or between the baseline and final intervention phases. The findings indicate that although short-term MD adherence is effective in improving certain microvascular physiological properties and QoL domains, there is room for additional improvement, observed in long-term adherers. Our findings are important in the design of future, MD-based, lifestyle interventions, with the advisable durations differing between target groups.