Smith's departure was widely observed by Boston Whigs, and several messages were slipped out of the city to alarm the countryside. After being alerted to the possibility that there was opposition on the road, Smith ordered some light infantry to move forward while he stayed with the main body of the expedition, thus he was not present at the skirmish in Lexington.

Major John Pitcairn of the Royal Marines was in charge of the advance guard. Pitcairn's men encountered a company of militiamen under John Parker on Lexington Green. Despite orders on both sides not to open fire, shooting broke out and a number of men were killed and wounded. By the time Smith arrived, his troops had scattered to pursue fleeing militiamen and it took some time for him to restore order and reform his companies. Several of his officers suggested that they should now return to Boston, but Smith decided to push on to Concord to complete the mission. He had sent messages back to Gage requesting reinforcements and informing him that the countryside was on alert.

Smith's conduct was highly praised by Gage in his dispatches to London reporting the outbreak of fighting. He was promoted to full colonel at the year's end and later promoted to brigadier general. On the night of 4 March 1776, during a snow storm, British sentries on duty near the Boston Neck heard digging across the bay on Dorchester Heights. This was reported to Smith, who did not report this to his own superiors at the Province House. By dawn on 5 March, the Americans now had a full breastworks constructed on the heights. This resulted in the British choosing to evacuate Boston and sail to Halifax, Nova Scotia.