Cornwallis was the governor-general of India and the commander in chief of defense forces from 1786 to 1793. He led a great campaign against the eradication of widespread corruption within the Indian administratve system. His reforms were in a way oppressive of the educated Indians, for he encouraged the Europeanization of Indian civil services, which reserved for the Europeans posts with an annual salary of over 500 pounds. He introduced a tax system that encouraged the local village landlords to collect the British taxes for a fee of 10 per cent. Cornwallis also created 23 separate administrative districts.

In 1792, Cornwallis famously defeated Tipu Sultan, the Indian ruler of Mysore, who had invaded Travancore, a state that was under British protection. Cornwallis commanded the combined forces of British, Maratha and Hyderabad against Tipu and forced him to cede nearly half of his territory.

After being made Marquess for his services in India, Cornwallis returned in 1805 for a second tenure as the governor-general. A few months after arriving in India, he succumbed to a fever, and died on 5 October 1805.