According to one source, Verelst came to Bengal in the service of the Company as early as 1749. Prior to the Battle of Plassey, he was taken prisoner by the ruler of Bengal Nawab Sirajuddowla. In June 1757, the Nawab was defeated at Plassey by the forces of Robert Clive, the event that marks the beginning of colonial rule in Bengal. Verelst was released from captivity once the battle was over. He served as a factor in Lakshmipur, and thereafter, in spite of his youth, he continued to achieve positions of responsibility. In 1758, he became a member of the Fort William Council. Two years later, he was sent to take control of the southern port of Chittagong, which had been ceded by the local ruler Mir Qasim.
Verelst ruled Chittagong for the next five years. His success in increasing tax revenues during this period was rewarded with further postings to Burdwan and Midnapur. In 1767, Robert Clive quit the governorship of Bengal. On 17 May of that year, the Company's Court of Directors chose Harry Verelst to replace him in Fort William. Verelst would occupy the post for the next two and a half years, until his resignation in December 1769.