By the summer of 1775, Grant returned to active service with the rank of colonel. He was ordered to return to North America. He arrived in Boston, Massachusetts, on 30 July. In the aftermath of the Battle of Bunker Hill, he urged General Gage to move the British troops to New York City, so that they would have room to manoeuvre. His advice was ignored at the time. Grant was a supernumerary until December when he was made colonel and commander of the 55th Regiment of Foot.

As the British tried to gain control of the major port city in the New York Campaign, Grant had become Howe's primary planning officer. He developed two plans, each designed to both gain control of territory and to deal a serious or fatal blow to the Continental Army. These resulted in the Battle of Brooklyn, and Battle of White Plains. Both of these were British victories, as was the overall campaign, but General Washington avoided the death blow to his troops each time.

Finally, Grant was shipped off to the West Indies. On 27 October 1778 he led a successful expeditionary force to capture the French West Indian island of St. Lucia. A superior French garrison, surrendered on 28 December, at the Battle of La Vigie.