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Aims: This study aims to evaluate the stability of C‐peptide over time and to compare fasting C‐peptide and C‐peptide response after mixed‐meal tolerance test (MMTT) at T90 or T120 with C‐peptide area under the curve (AUC) in long‐standing type 1 diabetes. Methods: We included 607 type 1 diabetes individuals with diabetes duration >5 years. C‐peptide concentrations (ultrasensitive assay) were collected in the fasting state, and in a subpopulation after MMTT (T0, just prior to, T30‐T60‐T90‐T120, 30–120 min after ingestion of mixed‐meal) (n = 168). Fasting C‐peptide concentrations (in n = 535) at Year 0 and Year 1 were compared. The clinical determinants associated with residual C‐peptide secretion and the correspondence of C‐peptide at MMTT T90 / T120 and total AUC were assessed. Results: A total of 153 participants (25%) had detectable fasting serum C‐peptide (i.e ≥ 3.8 pmol/L). Fasting C‐peptide was significantly lower at Year 1 (p < 0.001, effect size = −0.16). Participants with higher fasting C‐peptide had a higher age at diagnosis and shorter disease duration and were less frequently insulin pump users. Overall, 109 of 168 (65%) participants had both non‐detectable fasting and post‐meal serum C‐peptide concentrations. The T90 and T120 C‐peptide values at MMTT were concordant with total AUC. In 17 (10%) individuals, C‐peptide was only detectable at MMTT and not in the fasting state. Conclusions: Stimulated C‐peptide was detectable in an additional 10% of individuals compared with fasting in individuals with >5 years of diabetes duration. T90 and T120 MMTT measurements showed good concordance with the MMTT total AUC. Overall, there was a decrease of C‐peptide at 1‐year follow‐up. [ABSTRACT FROM AUTHOR]