The Boston campaign was the opening campaign of the American Revolutionary War, taking place primarily in the Province of Massachusetts Bay. The campaign began with the Battles of Lexington and Concord on April 19, 1775, in which the local colonial militias interdicted a British government attempt to seize military stores and leaders in Concord, Massachusetts. The entire British expedition suffered significant casualties during a running battle back to Charlestown against an ever-growing number of militia.

Subsequently, accumulated militia forces surrounded the city of Boston, beginning the siege of Boston. The main action during the siege, the Battle of Bunker Hill on June 17, 1775, was one of the bloodiest encounters of the war, and resulted in a Pyrrhic British victory. There were also numerous skirmishes near Boston and the coastal areas of Boston, resulting in loss of life, military supplies, or both.

In July 1775, George Washington took command of the assembled militia and transformed them into a more coherent army. On March 4, 1776, the colonial army fortified Dorchester Heights with cannon capable of reaching Boston and British ships in the harbor. The siege (and the campaign) ended on March 17, 1776, with the permanent withdrawal of British forces from Boston. To this day, Boston celebrates March 17 as Evacuation Day.

In 1767–68, the British Parliament passed the Townshend Acts, which imposed import duties on paper, glass, paint, and other common items imported into the American colonies. The Sons of Liberty and other Patriot organizations responded with a variety of protest actions. They organized boycotts of the goods subject to the duty, and they harassed and threatened the customs personnel who collected the duties, many of whom were either corrupt or related to Provincial leaders. Francis Bernard, then Governor of the Province of Massachusetts Bay, requested military forces to protect the King's personnel. In October 1768, British troops arrived in the city of Boston and occupied the city. Tensions led to the Boston Massacre on March 5, 1770, and the Boston Tea Party on December 16, 1773.

In response to the Tea Party and other protests, Parliament enacted the Intolerable Acts to punish the colonies. With the Massachusetts Government Act of 1774 it effectively abolished the provincial government of Massachusetts. General Thomas Gage, already the commander-in-chief of British troops in North America, was also appointed governor of Massachusetts and was instructed by King George's government to enforce royal authority in the troublesome colony. However, popular resistance compelled the newly appointed royal officials in Massachusetts to resign or to seek refuge in Boston. Gage commanded four regiments of British regulars (about 4,000 men) from his headquarters in Boston, but the countryside was largely controlled by Patriot sympathizers.

ON September 1, 1774, British soldiers removed gunpowder and other military supplies in a surprise raid on a powder magazine near Boston. This expedition alarmed the countryside, and thousands of American Patriots sprang into action, amid rumors that war was at hand. Although it proved to be a false alarm, this event—known as the Powder Alarm—caused all concerned to proceed more carefully in the days ahead, and essentially provided a "dress rehearsal" for events seven months later. Partly in response to this action, the colonists carried off military supplies from several forts in New England and distributed them among the local militias.

On the night of April 18, 1775, General Gage sent 700 men to seize munitions stored by the colonial militia at Concord. Several riders — including Paul Revere — alerted the countryside, and when the British troops entered Lexington on the morning of April 19, they found 77 minutemen formed up on the village common. Shots were exchanged, eight Minutemen were killed, the outnumbered colonial militia dispersed, and the British moved on to Concord. At Concord, the troops searched for military supplies, but found relatively little, as the colonists, having received warnings that such an expedition might happen, had taken steps to hide many of the supplies. During the search, there was a confrontation at the North Bridge. A small company of British troops fired on a much larger column of colonial militia, which returned fire, and eventually routed the British, who returned to the village center and rejoined the other troops there. By the time the "redcoats" or "lobster backs" (as the British soldiers were called) began the return march to Boston, several thousand militiamen had gathered along the road. A running fight ensued, and the British detachment suffered heavily before reaching Charlestown. With the Battle of Lexington and Concord — the "shot heard 'round the world" — the war had begun.

In the aftermath of the failed Concord expedition, the thousands of militiamen that had converged on Boston remained. Over the next few days, more arrived from further afield, including companies from New Hampshire, Connecticut, and Rhode Island. Under the command of Artemas Ward, they surrounded the city, blocking its land approaches and putting the occupied city under siege. The British regulars fortified the high points in the city.