(1) Background: Concomitant mild aortic valve disease is frequently found in patients undergoing rheumatic mitral valve surgery. To date, only a limited number of single-center studies have specifically addressed the untreated baseline aortic valve disease long-term progression and reoperation rate. Thus, we conducted a meta and landmark analysis to systematically review the issue. (2) Methods: This study investigated the long-term prognostic of baseline mild aortic valve disease in patients undergoing rheumatic mitral valve surgery, based on evidence from PubMed, Embase, Cochrane Library, and Web of Science databases. (3) Results: Meta analysis revealed that patients with mild aortic valve disease had a higher risk of disease progression, with a 3.3-fold risk in the 0–5-year follow-up, which jumped to a hazard ratio of 6.42 in longer-term follow-up (5–25 years). Patients with aortic stenosis had an 8.37-fold risk of progression compared with aortic regurgitation and appeared to be poorly related to the time cut-off. Similarly, higher reoperation rates at long-term follow-up were seen in aortic stenosis patients. (4) Conclusions: This study suggests that patients with mild aortic valve disease at baseline have poorer long-term aortic valve-related progression and reoperation rates, especially aortic stenosis. For those with concomitant aortic stenosis, further investigation of the impact of lesion progression is warranted.