High premature adult deaths in developing countries are gaining attention, as recent studies show their increasing impact on overall mortality rates. This paper has twofold objectives: firstly, it investigates the long-term trends and patterns of adult mortality between 1970 and 2018 in India. Secondly, it attempts to detect age, period, and cohort (APC) effects on adult mortality decline over time. Data on age-specific mortality rates and disease-adjusted life years for adult age groups (15–59 years) were collected from the Sample Registration System and the Global Burden of Disease study, respectively. The trends in age-standardized mortality rates were presented graphically, and critical change points were highlighted using a change-point analysis. The intrinsic estimator model was applied to estimate the independent effects of APC on adult mortality. The findings revealed that adult mortality declined between 1970 and 2018 with multiple critical change points. The APC effects showed a notable decline in adult mortality during 2005–2018 and for the recent birth cohorts, 1980–2004. However, the rate of mortality declined slowly over time. Results also indicated that mortality started increasing from mid-adult ages and peaked in older adult ages due to the age effects and provided evidence of a rise in adult life loss due to non-communicable diseases in recent years. Overall, the study underscores the importance of implementing health policies aimed at reducing life loss in the most economically active ages that can have long-term negative implications for the country’s economic growth.