Type 2 diabetes mellitus (T2DM) as a comorbidity in amyotrophic lateral sclerosis (ALS) has sparked interest for its potential impact on disease expression and prognosis. In this retrospective cohort study, we investigated the prevalence and clinical correlates of T2DM in a large cohort of patients from the ALS registry of a Northern Italy region, Emilia Romagna, established in 2009. Out of 1756 ALS patients enrolled up to 2021, 145 were affected by T2DM (diALS). Patients with diALS were older than those without T2DM (ndALS) (71.56 vs. 65.76 years, p < 0.001), had a higher body mass index (25.63 vs. 24.23, p < 0.001), but experienced greater weight loss at diagnosis (6.87% vs. 5.44%, p < 0.007). Respiratory onset (6.2% vs. 2.6%, p = 0.013) and respiratory phenotype (4.2% vs. 1.4%, p = 0.04) were more frequent among diALS. Coherently, diALS presented a lower forced vital capacity (74.9% vs. 87.9%, p ≤ 0.001) and more frequently adopted Non-Invasive Ventilation (NIV) (50.35% vs. 37.61%, p = 0.003), with significant influence on time to NIV (HR 1.71, 95% CI 1.07–2.74, p = 0.024). Exploring genetic background, among all the genes examined C9ORF72 emerged as underrepresented among diALS (7.64% in ndALS vs. 0% in diALS, p = 0.039). In conclusion, we confirmed a more severe respiratory dysfunction in diALS, suggesting a specific frailty in respiratory muscles, together with some peculiar clinical features consistent with the previous literature data, such as a later onset. The lower prevalence of C9ORF72 expansion in this population may hint towards a specific role of the gene in metabolism and inflammation, granting more space to non-genetic causes, warranting further studies for confirmation.