With the outbreak of the American Revolutionary War with the Battles of Lexington and Concord in April 1775, Lovell's military activity increased. He was commissioned a colonel of the 2nd Massachusetts Regiment in February 1776, and his troops were among those that occupied the heights south of Boston, prompting the British to withdraw the city. He continued to be active in the defense of eastern Massachusetts, and was promoted to brigadier general of the Suffolk County militia on June 24, 1777. Lovell led Massachusetts troops in the 1778 Battle of Rhode Island, where Lovell was one of several officers who "distinguished themselves by their coolness and courage."