Date: April 19, 1775 - March 17, 1776 The Siege of Boston began right after the Battles of Lexington and Concord. The Colonial militia who had gathered to stop the British raids followed the British army to Boston... and camped outside, bottling British troops and their Loyalist allies in the city. At the time, Boston was a peninsula, attached to the mainland by a thin strip of land known as Boston Neck. With this blocked, land access to the city was cut off.