Diabetes mellitus (DM) is a chronic disease, which requires optimal glycemic control to prevent its ensuing vascular complications. Pathway to optimal glycemic control in T2DM has a complex socio-behavioral construct, especially in vulnerable populations, like slum dwellers, who have reduced health-care access and lower prioritization of health needs. The study aimed to map trajectories of glycemic control amongst individuals with T2DM living in urban slums and identify key determinants associated with unfavourable glycaemic trajectory. This study was a community-based longitudinal study conducted in an urban slum of Bhopal in Central India. Adult patients diagnosed with T2DM and on treatment for more than one year were included. All 326 eligible participants underwent a baseline interview, which captured sociodemographic, personal behavior, medication adherence, morbidity profile, treatment modality, anthropometric and biochemical measurements (HbA1c). Another 6-month follow-up interview was conducted to record anthropometric measurements, HbA1c and treatment modality. Four mixed effect logistic regression models (through theory-driven variable selections) were created with glycemic status as dependent variable and usage of insulin was considered as random effect. A total of 231 (70.9%) individuals had unfavorable glycemic control trajectory (UGCT), and only 95 (29.1%) had a favorable trajectory. Individuals with UGCT were more likely to be women, with lower educational status, non-vegetarian food preference, consumed tobacco, had poor drug adherence, and were on insulin. The most parsimonious model identified female gender (2.44,1.33-4.37), tobacco use (3.80,1.92 to 7.54), and non-vegetarian food preference (2.29,1.27 to 4.13) to be associated with UGCT. Individuals with good medication adherence (0.35,0.13 to 0.95) and higher education status (0.37,0.16 to 0.86) were found to be protective in nature. Unfavorable glycemic control trajectory seems to be an inescapable consequence in vulnerable settings. The identified predictors through this longitudinal study may offer a cue for recognizing a rational response at societal level and adopting strategy formulation thereof.