Nearly half of women with gestational diabetes mellitus (GDM) progress to type 2 diabetes mellitus (T2DM) within 10 years after pregnancy. However, susceptibility to T2DM as well as time to progression after GDM pregnancy varies individually. The clinical and genetic factors determining this variability are not fully understood yet. We investigated the antepartum clinical, metabolic, and genetic differences among early converters (who were diagnosed as T2DM at postpartum 2 months), late converters (who progressed to T2DM more than 1 years later), and non-converters after GDM pregnancy. The study was a prospective longitudinal cohort study involving 843 Korean women with GDM pregnancy. Oral glucose tolerance test (OGTT) was performed at 2 months postpartum and annually for median of 47 months. Study subjects were genotyped for 21 genetic variants in PPARG, IGF2BP2, CDKAL1, SLC30A8, CDKN2A/2B, HHEX, TCF7L2, KCNQ1, KCNJ11, and FTO. Among the 843 GDM women, 105 (12.5%) progressed to T2DM early postpartum. Among the 418 women who had follow up visits for more than one year, 116 (27.8%) progressed to diabetes. Increased body mass index, family history of T2DM were significantly associated with risk of T2DM. In a sub-study involving 86 GDM women who undertook abdominal fat CT, increased visceral-to-subcutaneous fat ratio was associated with development of T2DM. Early converters had higher glucose and lower insulin concentration during diagnostic OGTT performed at pregnancy compared to late converters. The early converters had significantly decreased 1-hour insulinogenic index compared to late converters. Genetic variants in HHEX were associated with early conversion to diabetes and variants in CDKAL1 were associated with late conversion to diabetes. In conclusion, we found that a significant proportion of GDM women progress to T2DM after parturition and women who progress to T2DM early post-partum have decreased insulin secretory capacity compared to late converters. These differences might be explained in part by genetic predisposition.