Britain responded harshly to the Boston Tea Party and effectively closed the Port of Boston to outside trade and rigged the Massachusetts Council with royal appointees. However, contrary to earlier actions, Bostonians showed few signs of weakening in the face of harsh British treatment. Adams's long-time ploy of provoking the British to the point of punishing the colonies was paying off. Every punishment and tribulation now only underscored Adams's warnings of tyranny and oppression and helped to unite the colonies. Adams crafted the "Solemn League and Covenant," which suspended all colonial trade with Britain until the restrictions on the Boston port were lifted. The sweeping agreement went far beyond earlier nonimportation agreements, and the citizens of Massachusetts were sure that the British bankers–who had extended more than four million pounds' worth of loans to colonists–would not let the British government throw the colonies away. Adams knew such an agreement would force Britain to act. Although heartily adopted in Massachusetts, the agreement met resistance in other colonies. Adams had seriously misjudged his national support, particularly that of the middle and southern colonies. New York countered with the idea of a Continental Congress, where the various colonies could meet to draw up a list of grievances and adopt a "national" policy. Adams gladly agreed, as he had been urging such a meeting since 1773.

When Massachusetts voted to send delegates to the Congress, Adams proudly joined the delegation. Still remarkably poor, his friends first outfitted him with "dressy" clothes (including a new wig) and paid for his travel to Philadelphia. The delegates traveled in comfortable style to Philadelphia and along the way met a hearty reception: church bells and cannons greeted them in Connecticut and the New York merchants treated the group to the best dinner John Adams had ever seen.

However nice the journey, the delegates met with hostility from many other delegates once they arrived. Several colonies, most of them southern, feared that Massachusetts would attempt to seize control of the colonies if Britain left. The Bostonians' dogma was far advanced from that of the south and mid- Atlantic colonies. However, their fears expressed only an ignorance of Samuel Adams, since much of his political theory stemmed from Locke, who had argued that the purpose of government was to protect property. Thus, Adams and the other Massachusetts delegates tried to alleviate the concerns of the other delegates by lying low through the early days of the convention. Adams worked closely behind the scenes with Virginian Richard Henry Lee and Christopher Gadsden of South Carolina to carry out the intended course of the Congress. The patriots' most determined opponent, though, was Joseph Galloway of Pennsylvania. On September 28, 1774, Galloway, while acknowledging the power of Parliament, introduced his plan for America and Britain to form a strong "political union." In Galloway's plan, the colonies would be governed by an American Grand Council, the equivalent of a local Parliament, and a Crown-appointed Resident General would oversee the Council. By a vote of six to five, the Congress agreed to continue the bill's deliberations the following day. But Adams stopped it cold that night, by inflaming a Philadelphia mob to the point that Galloway feared for life if the bill came up again.

Before leaving for the Congress, Adams had asked that his supporters in Massachusetts rally and pass resolutions in protest of Britain's policies. When the Suffolk Resolves–which denied the legitimacy of the Port Bill, urged defensive preparations and ordered committees of correspondence to ready militias–arrived in Philadelphia it took a long, hot debate before the Congress would agree to them. The resolves became the equivalent of a mutual defense clause, and it implied that an attack on Boston would be responded to with military force from across the colonies. The Congress also passed a less- sweeping version of the Solemn Covenant, but none doubted the measures would still have an effect. Richard Henry Lee wrote that he believed the ship that carried news of the Congress to England would return with a promise of compromise and reconciliation by the Crown.

Anti-British tempers flared throughout the colonies. When a rumor announced that six men had been killed by British troops in Boston, thousands of armed men from Connecticut, Rhode Island, and New York set off to drive the British armies back into the sea–only to turn back when word reached them that the rumor was false. Meanwhile, Adams returned to Boston to find it radically changed. Tories from across the area had fled into the city rather than take their chances in the patriotic countryside. Suddenly Boston, guarded by British troops and men-of-war, had become a Tory safe-haven. Orders arrived for the British troops to find and arrest Adams and other leading patriots, but Adams fled safely to Lexington along with John Hancock before the British could locate him.

On April 18, 1775, British troops marched on Lexington and Concord in an attempt to locate supposed stores of weaponry that had been massed there for possible rebellion. Alerted by several riders, including Paul Revere, colonial militiamen lay in wait for the troops. Fighting broke out on the Lexington Green, and Adams–hiding nearby–remarked, "Oh, what a glorious morning is this!" The redcoats fought a running battle with the Continental soldiers, called Minutemen, all the way back to Boston.

As war began, the Continental Congress met to chose a commanding general for the fledgling military. George Washington, a delegate from Virginia, was the only member of the group to appear in a military uniform, and thus after much hemming and hawing, John Adams placed his name in nomination for the post. Sam Adams had much wanted to see a New Englander in charge of the army but realized that such a move was unpopular among the southerners. When Sam Adams rose to second Washington's nomination, Hancock turned pale. He had long hoped to be the commander, and the split ended his friendship with both Adamses for the remainder of the war.

The next issue for the Congress proved to be more divisive. Both Adamses now pushed for the Congress to declare independence from Britain, a move that the southern conservatives were loathe to make. Over the coming months, John and Sam Adams made many enemies in the Congress, and for a while, John Adams found himself almost an outcast. They argued that the tardiness in declaring independence made the colonies seem weak and helped strengthen Tory resolve. Instead, the Congress waited until the British attacks had grown to such a point where it was forced to declare independence. By then, Sam Adams was recognized across the colonies as the leader of the revolutionaries. Thomas Jefferson called him the "Man of the Revolution," and one Tory called him the "Machiavelli of Chaos." More popularly, though, he had become known as the "Father of America." However popular or infamous he might have been across the country, though, Adams showed a marked lack of statesmanship. His career in the Congress was marred by missteps, fights, and scandals, and he never achieved the level of recognition that other members did, even of his cousin John.