Background/Objectives: Twins discordant for Parkinson’s disease (PD) share genetic background and early-life exposures, offering a model for non-genetic determinants of disease. We aimed to evaluate longitudinally the mortality, clinical and lifestyle features, nutritional status and dietary habits of a cohort of PD-discordant twins, with specific attention to the monozygotic (MZ) subgroup. Methods: One hundred and eight subjects from 54 PD-discordant twin pairs (26 MZ, 28 dizygotic [DZ]), with genetically confirmed zygosity, were assessed at baseline (T0) and after 10 years by telephone interview (T1; 33 pairs), using Kaplan–Meier/Cox survival analyses, a clinical and lifestyle history questionnaire, body mass index (BMI), and a food frequency questionnaire (FFQ). Results: Ten-year mortality was markedly higher in PD (31.5% vs. 5.6%; HR 6.83), with all MZ deaths occurring in the affected twin. Constipation was more frequent in PD at both time points (T0 p = 0.004; T1 p = 0.011). Comorbidities, smoking, weight and BMI did not differ within pairs. Preserved-fish intake was stably lower in PD (T0 p = 0.021; T1 p = 0.004); fruit intake was lower in PD at baseline (MZ, p = 0.001) but higher at follow-up (p = 0.029). Conclusions: The mortality difference persisted despite no detected within-pair differences in BMI and comorbidities, consistent with a major contribution of PD itself. Preserved-fish intake remained stably lower in affected twins, whereas increased fruit intake at follow-up may reflect adaptation to constipation; these exploratory dietary findings warrant confirmation.