Background Cortical lesions develop early in multiple sclerosis (MS) and play a major role in disease progression. MRI at 7.0 T shows high sensitivity for detection of cortical lesions as well as better spatial resolution and signal-to-noise ratio compared with lower field strengths. Purpose To longitudinally characterize (a) the development and evolution of cortical lesions in multiple sclerosis across the cortical width, sulci, and gyri; (b) their relation with white matter lesion accrual; and (c) the contribution of 7.0-T cortical and white matter lesion load and cortical thickness to neurologic disability. Materials and Methods Twenty participants with relapsing-remitting MS and 13 with secondary progressive MS, along with 10 age-matched healthy controls, were prospectively recruited from 2010 to 2016 to acquire, in two imaging sessions (mean interval, 1.5 years), 7.0-T MRI T2*-weighted gradient-echo images (0.33 × 0.33 × 1.0 mm3) for cortical and white matter lesion segmentation and 3.0-T T1-weighted images for cortical surface reconstruction and cortical thickness estimation. Cortical lesions were sampled through the cortex to quantify cortical lesion distribution. The Expanded Disability Status Scale (EDSS) was used to assess neurologic disability. Nonparametric statistics assessed differences between and within groups in MRI metrics of cortical and white matter lesion burden; regression analysis explored associations of disability with MRI metrics. Results Twenty-five of 31 (81%) participants developed new cortical lesions per year (intracortical, 1.3 ± 1.7 vs leukocortical, 0.7 ± 1.9; P = .04), surpassing white matter lesion accrual (cortical, 2.0 ± 2.8 vs white matter, 0.7 ± 0.6; P = .01). In contrast to white matter lesions, cortical lesion accrual was greater in participants with secondary progressive MS than with relapsing-remitting MS (3.6 lesions/year ± 4.2 vs 1.1 lesions/year ± 0.9, respectively; P = .03) and preferentially localized in sulci. Total cortical lesion volume independently predicted baseline EDSS (β = 1.5, P < .001) and EDSS changes at follow-up (β = 0.5, P = .003). Conclusion Cortical lesions predominantly develop intracortically and within sulci, suggesting an inflammatory cerebrospinal fluid–mediated lesion pathogenesis. Cortical lesion accumulation was prominent at 7.0 T and independently predicted neurologic disability progression. © RSNA, 2019 Online supplemental material is available for this article.