Background: The EAT-Lancet commission proposed, in 2019, a planetary, healthy, and universal dietary pattern. However, this diet has rarely been studied in relation to various health outcomes. Objectives: We aimed to prospectively estimate the association between the EAT-Lancet diet and cancer and cardiovascular disease (CVD) risk. Methods: The study was conducted in participants of the NutriNet-Santé cohort (2009–2021). The endpoints were the incident outcomes (cancer and CVDs and mortality from these diseases), combined and separately. Adherence to the EAT-Lancet diet was estimated using the EAT-Lancet Diet Index (ELD-I) modeled as quintiles (Qs). Multivariable Cox proportional hazard models were used to estimate HRs and 95% CIs, adjusted for potential confounders and moderators. Results: A total of 62,382 subjects were included; 2475 cases of cancer and 786 cases of cardiovascular events occurred during a median follow-up of 8.1 y. The sample was 76% female, the mean ± SD age at inclusion was 51.0 ± 10.2 y. The ELD-I ranged from −162 to 332 points with a mean ± SD score of 45.4 ± 25.6 points. In multivariable models, no significant association between the EAT-Lancet diet and the risk of cancer and CVD combined, and separately, was observed. Alcohol consumption was an effect modifier of the association. A significant association was observed among low drinkers (HRQ5vs.Q1: 0.86; 95% CI: 0.73, 1.02; P-trend = 0.02). A higher ELD-I was significantly associated with a lower risk of overall cancer only among females (HRQ5vs.Q1: 0.89; 95% CI: 0.75, 1.05; P-trend = 0.03). Both associations were largely attenuated by BMI. Conclusions: Contrary to our hypothesis, our results documented significant associations between adherence to the EAT-Lancet diet and incidence of cancer only in some subgroups, and no association with CVD.