Low tide opened a gap along the Mystic River to the north, so they quickly extended the fence with a short stone wall to the water's edge. Colonel Stark placed a stake about 100 feet (30 m) in front of the fence and ordered that no one fire until the British regulars passed it.

The British had taken the ground but at a great loss; they had suffered 1,054 casualties (226 dead and 828 wounded), and a disproportionate number of these were officers. The casualty count was the highest suffered by the British in any single encounter during the entire war. General Clinton echoed Pyrrhus of Epirus, remarking in his diary that "A few more such victories would have shortly put an end to British dominion in America." British dead and wounded included 100 commissioned officers, a significant portion of the British officer corps in America. Much of General Howe's field staff was among the casualties

Colonial losses were about 450 in total, of whom 140 were killed. Most of the colonial losses came during the withdrawal. Major Andrew McClary was technically the highest ranking colonial officer to die in the battle; he was hit by cannon fire on Charlestown Neck, the last person to be killed in the battle. He was later commemorated by the dedication of Fort McClary in Kittery, Maine.

The Massachusetts Committee of Safety sought to repeat the sort of propaganda victory that it won following the battles at Lexington and Concord, so it commissioned a report of the battle to send to England. Their report, however, did not reach England before Gage's official account arrived on July 20. His report unsurprisingly caused friction and argument between the Tories and the Whigs, but the casualty counts alarmed the military establishment, and forced many to rethink their views of colonial military capability.

King George's attitude hardened toward the colonies, and the news may have contributed to his rejection of the Continental Congress' Olive Branch Petition, the last substantive political attempt at reconciliation. Sir James Adolphus Oughton, part of the Tory majority, wrote to Lord Dartmouth of the colonies, "the sooner they are made to Taste Distress the sooner will [Crown control over them] be produced, and the Effusion of Blood be put a stop to." About a month after receiving Gage's report, the Proclamation of Rebellion was issued in response. This hardening of the British position also strengthened previously weak support for independence among Americans, especially in the southern colonies

Warren was commissioned into the Continental Army at the rank of major general by the Second Continental Congress on June 14, 1775. Three days later, he arrived at Charlestown just before the battle of Bunker Hill began and made his way to where Patriot militiamen were forming. Upon meeting General Israel Putnam, Warren asked where he thought the heaviest fighting would be; Putnam responded by pointing to Breed's Hill. Warren subsequently volunteered to join the militia at the rank of private against the wishes of both Putnam and Colonel William Prescott, both of whom unsuccessfully requested that he serve as their commander instead. Warren declined their request due the fact that Putnam and Prescott held more military experience.

"Wherever danger is, dear mother," he answered, "there will your son be. Now is no time for one of America's children to shrink from the most hazardous duty; I will either set my country free, or shed my last drop of blood to make her so."