Background: We aimed to assess the long-term effects of the introduction of continuous subcutaneous insulin infusion (CSII) and continuous glucose monitoring (CGM) in people with type 1 diabetes (T1D). Methods: A prospective single-centre cohort study including participants with T1D and HbA1c > 7.5%. After completing a course in flexible intensified insulin treatment (FIT), participants were offered treatment change to CSII/CGM. FIT participants with HbA1c ≤ 7.5% who remained on multiple daily injections (MDI) and without CGM were monitored as a separate cohort to compare the cumulative incidence of diabetic complications. Results: The study cohort included 41 participants with T1D (21 male/20 female). The mean age (±SD) at inclusion was 24.2 ± 10.9 years, the mean follow-up was 8.9 ± 2.8 years, and the mean diabetes duration at the end of the study was 15.9 ± 10.1 years. The mean HbA1c level before the introduction of CSII was 8.8 ± 1.3% (73 ± 8 mmol/mol), and decreased significantly thereafter to 8.0 ± 1.1% (63 ± 7 mmol/mol) (p = 0.0001), and further to 7.6 ± 1.1% (59 ± 11 mmol/mol) after the initiation of CGM (p = 0.051). In the MDI group the HbA1c levels did not change significantly during a mean follow-up of 6.8 ± 3.2 years. The frequency of severe hypoglycaemia after the introduction of CSII/CGM declined significantly (from 9.7 to 2.2 per 100 patient-years, p = 0.03), and the cumulative incidence of newly diagnosed diabetic microvascular complications were comparable between the study group and the observational cohort. Conclusion: In people with T1D and unsatisfactory diabetes control the introduction of CSII and CGM results in a substantial and long-term improvement.