Abstract Context Glycemic control in adolescents with type 1 diabetes is poor; yet, it typically improves during early adulthood. Factors related to improvement of glycemic control are unclear. Objective This work examines how demographic and clinical variables may affect trajectories of glycemic control over time. Methods This retrospective, observational study comprised 1775 participants ages 18 to 30 years at enrollment in the T1D Exchange clinic registry. Latent class trajectory modeling was used to determine subgroups following a similar glycated hemoglobin A1c (HbA1c) trajectory over time. Results Five distinct trajectories of HbA1c classes were identified: “low-decline” and “moderate-decline” groups had low or moderate HbA1c with a gradual decline, the “high-stable” group had high HbA1c and remained stable, and the “very high-rapid decline” and “very high-slow decline” groups had very high HbA1c with rapid or gradual decline. Compared with the “high-stable” group, the “low-decline” and “moderate-decline” groups were more likely to be male (P = .009), White non-Hispanic (P = .02), nonsmokers (P < .001), check self-monitoring blood glucose (SMBG) more frequently (P < .001), and have higher education (P < .001), lower body mass index (P = .02), and lower daily insulin dose (P < .001). Compared with the “very high-rapid decline” and “very high-slow decline” groups, the “low-decline” and “moderate-decline” groups were more likely to be male (P = .02), have higher education (P < .001), use insulin pumps (P = .01), be nonsmokers (P < .001), and have a higher number of SMBG checks per day at enrollment (P < .001). Conclusion We determined 5 distinct patterns of glycemic control from early adulthood into adulthood. Further evaluation into the modifiable factors associated with a declining HbA1c trajectory would aid in the development of targeted interventions.