Modern indications for medial mobile-bearing unicompartmental knee arthroplasty (UKA) include a normal lateral compartment, minimal patellofemoral disease, and a ligamentously stable knee. Radiographs and intraoperative inspection can determine the appropriateness of UKA. Magnetic resonance imaging (MRI) interpretations can over-estimate the degree of knee pathology. This study reports the outcomes of UKA performed despite an abnormal MRI of the lateral compartment, patellofemoral compartment, and/or cruciate ligaments. One thousand consecutive medial UKAs were reviewed, and 33 patients had pre-operative MRI with interpretations of osteoarthritic changes in the lateral compartment, patellofemoral compartment, and/or deficiency of the anterior cruciate ligament (ACL). We compared the postoperative Knee Society pain score, total score, and functional score between the abnormal MRI group (n = 33) and the remaining patients (n = 967). Average follow-up was 43.4 months and 38.3 months for the two groups, respectively. Knee Society pain, total, and functional scores for the abnormal MRI group were 40.8, 88.7, and 78.5 respectively compared with 43.4, 90.6, and 80.0 respectively for the remaining patients. The failure rate was 3% (1/33) in the abnormal MRI group and 4% (39/967) in the remaining patients. Based on the numbers available, there were no differences between the two groups in terms of survival and clinical results. The results of this study suggest abnormal preoperative MRI findings do not have an influence on the outcome of UKA when modern radiographic and clinical criteria are met.