Rennell entered the Royal navy as a midshipman at the age of fourteen under Captain Hyde Parker, who was in the process of commissioning the frigate (launched in October 1757) at the beginning of the Seven Years' War. He was present at the Raid on Cherbourg (1758), and at the disastrous Battle of Saint Cast in the same year. In 1760, he went out to East India, and served in under Captain Hyde Parker during the three following years, when he saw some active service, including a cutting-out expedition at Pondicherry. He soon mastered the theory and practice of marine surveying, and, on account of his proficiency in this regard, Parker lent his services to the East India Company. He served for a year on board one of the company's ships bound to the Philippines, with the object "of establishing new branches of trade with the natives of the intervening places". Rennell accompanied the hydrographer Alexander Dalrymple and drew several charts and plans of harbours on journeys in the schooner "Cuddalore" (1759–62), the "London" (1762-63) and the "Neptune" (1763–64).