Background/Objectives: Humeral head osteonecrosis is a recognised complication following locked plate fixation of complex proximal humeral fractures, yet the clinical relevance of necrotic lesion extent remains poorly characterised. This study aimed to quantify MRI-defined osteonecrosis extent and examine its associations with radiographic severity, shoulder motion, and functional outcomes. Methods: Twenty-five patients with Neer three- or four-part proximal humeral fractures treated by locked plate fixation between 2010 and 2022 were retrospectively reviewed. Active shoulder range of motion, Constant score, American Shoulder and Elbow Surgeons (ASES) score, and visual analogue scale (VAS) pain were assessed at final follow-up. Osteonecrosis severity was graded using the Cruess classification. MRI necrotic extent was quantified with a modified angle-based method on coronal and axial images; associations with outcomes were assessed by linear regression. Results: At a mean follow-up of 85.4 ± 34.1 months, osteonecrosis was identified in 12 of 25 patients (48%). Patients with osteonecrosis demonstrated significantly lower shoulder abduction (96.7 ± 27.1° vs. 121.8 ± 29.2°; p = 0.031), internal rotation (48.3 ± 15.9° vs. 66.4 ± 15.5°; p = 0.014), and external rotation (36.7 ± 16.7° vs. 51.8 ± 26.4°; p = 0.041) than those without. Functional outcome scores did not differ significantly between groups. Greater total necrotic angle was associated with higher radiographic Cruess stage and reduced shoulder abduction (β = −0.141; 95% CI −0.259 to −0.024; p = 0.020). Conclusions: MRI-defined osteonecrosis extent correlates with radiographic severity and reduced shoulder motion after locked plate fixation of proximal humeral fractures. Larger necrotic lesions are associated with decreased shoulder abduction, whereas no clear relationship was identified between necrotic extent and global functional outcome scores. Quantitative MRI assessment may provide additional clinically relevant information on the structural severity of post-traumatic humeral head osteonecrosis.