Exposure to ambient fine particulate matter (PM2.5) is identified as one of the leading risk factors for morbidity and mortality in India. Here we estimate ambient PM2.5 exposure and its 18-year (1998–2015) trend in 109 Indian cities using satellite data and further classify them into six (index 1 for low vulnerability to 6 for extreme vulnerability) vulnerable classes. PM2.5 exposure shows a rapid increase in Delhi and the cities in Uttar Pradesh, Bihar, Jharkhand, West Bengal, Punjab, Haryana, Rajasthan, Madhya Pradesh, Chhattisgarh and Odisha. Amongst the cities with population more than 0.5 million (as per 2011 census), Thiruvananthapuram is the least vulnerable and Aligarh is the most vulnerable city based on 18-year statistics. Only 27 cities are identified as 'low' to 'moderately' vulnerable to ambient air pollution. The median incremental rate of annual PM2.5 exposure increases by 57.9% (from 0.9 to 1.15 µg m–3 per year) with the night-light counts (a proxy for urbanization rate) increasing from < 20th percentile to > 80th percentile. 51 out of the 60 Indian cities chosen for the 'smart city' mission are highly vulnerable to PM2.5 exposure (vulnerability index > 2), thereby facing a challenge to achieve the core objective (i.e., sustainable environment) of the mission. Our results will facilitate prioritizing clean-air action plan for the cities based on their vulnerability rankings to achieve maximum health benefit of the exposed population.