OBJECTIVE: To investigate the trends in diabetes prevalence, diagnosis, and management among Mexican adults who were participants in a long-term prospective study. RESEARCH DESIGN AND METHODS: From 1998 to 2004, 159,755 adults from Mexico City were recruited to a prospective study, and from 2015 to 2019, 10,144 survivors were resurveyed. Diabetes was defined as self-reported diagnosis, glucose-lowering medication use, or HbA1c ≥6.5%. Controlled diabetes was defined as HbA1c <7%. Prevalence estimates were uniformly standardized for age, sex, and residential district. Cox models explored the relevance of controlled and inadequately controlled diabetes to cause-specific mortality. RESULTS: During 1998-2004 and 2015-2019, 99,623 and 8,986 participants were aged 45-84 years. Diabetes prevalence had increased from 26% in 1998-2004 to 35% by 2015-2019. Of those with diabetes, the proportion previously diagnosed had increased from 76 to 89%, and glucose-lowering medication use among them had increased from 80 to 94%. Median HbA1c among those with diabetes had decreased from 8.2 to 7.3%, and the proportion of participants with controlled diabetes had increased from 16 to 37%. Use of blood pressure-lowering medication among those with previously diagnosed diabetes had increased from 35 to 51%, and their use of lipid-lowering therapy had increased from 1 to 14%. The excess mortality risk associated with diabetes accounted for 34% of deaths at ages 35-74 years, of which 5% were attributable to controlled and 29% to inadequately controlled diabetes. CONCLUSIONS: Inadequately controlled diabetes is a leading cause of premature adult death in Mexico. Improvements in diabetes management have increased diagnosis and control, but substantial opportunities remain to improve treatment, particularly with lipid-lowering therapy.