Maturity-onset diabetes of the young (MODY) is a highly heterogeneous group of monogenic and nonautoimmune diseases. Misdiagnosis of MODY is a widespread problem and about 5% of patients with type 2 diabetes mellitus and nearly 10% with type 1 diabetes mellitus may actually have MODY. Using next-generation DNA sequencing (NGS) to facilitate accurate diagnosis of MODY, this study investigated mutations in 13 MODY genes (, , , , , , , , , , , , and ). In addition, we comprehensively investigated the clinical phenotypic effects of the genetic variations identified. Fifty-one adult patients with suspected MODY and 64 healthy controls participated in the study. We identified 7 novel and 10 known missense mutations localized in , , , , , and genes in 29.4% of the patient sample. Importantly, we report several mutations that were classified as "deleterious" as well as those predicted as "benign." Notably, the p.R1103Q, p.V421I, I336T, p.N493H, p.L503P, p.S362P, and p.E69A mutations were identified for the first time as causative variants for MODY. More aggressive clinical features were observed in three patients with double- and triple-heterozygosity of - (p.E69A/p.S182R), -- (p.I336, p.G157R/p.R1103Q/p.A157A), and - (p.S362P/p.P261L). Interestingly, the clinical effects of the mutations appear to be exacerbated in the presence of obesity. In conclusion, NGS analyses of the adult patients with suspected MODY appear to be informative in a clinical context. These findings warrant further clinical diagnostic research and development in different world populations suffering from diabetes with genetic underpinnings.