Background: Unicompartmental knee arthroplasty (UKA) represents a well-recognized treatment option for isolated medial compartment osteoarthritis; however, the debate regarding the superiority of fixed-bearing versus mobile-bearing designs continues. We aimed to evaluate the mid- to long-term outcomes of medial UKA comparing mobile- versus fixed-bearing designs within a single institution over an average 10-year follow-up. Methods: This retrospective study included 81 consecutive patients who underwent primary medial UKA (45 fixed-bearing and 36 mobile-bearing) with a minimum five-year follow-up. Clinical outcomes were measured using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score and range of motion (ROM). Radiological measurements included hip-knee-ankle axis angle (HKA) and osteoarthritis progression. Implant survivorship was evaluated using Kaplan–Meier analysis, with failure defined as either conversion to total knee arthroplasty (TKA) or polyethylene (PE) exchange. Results: At a mean follow-up of 10.6 years, WOMAC scores, ROM, and radiological outcomes showed no statistically significant differences between the fixed-bearing and mobile-bearing groups. Significantly higher failure rates were observed in the mobile-bearing group, both when considering conversion only (p = 0.041) and when including conversion or PE exchange (p = 0.009). Survival analysis demonstrated 10-year rates of 97.8% for fixed-bearing and 88.9% for mobile-bearing with TKA conversion defined as failure (p = 0.066). Using combined failure criteria of TKA conversion or PE exchange, 10-year survival rates were 97.8% for fixed-bearing and 83.3% for mobile-bearing (p = 0.015). Conclusions: At a mean 10.6-year follow-up, clinical and radiological outcomes were comparable, but fixed-bearing UKA demonstrated superior survivorship.