Franklin was at sea, on his way home from London, when the Battles of Concord and Lexington broke out. By the time he landed in Philadelphia, the colonies were at war with Britain. Without missing a beat, Franklin embraced the Revolutionary War. The day after his arrival, he was unanimously elected as a delegate to the Second Continental Congress. He immediately became one of the most radical members of the Congress, proposing a unified American government that would have given the central government even greater powers than the United States Constitution eventually did. From March to May of 1776, Franklin participated in a mission to Canada, where he unsuccessfully tried to convince colonists there to join the revolt against Britain. Back in Philadelphia that summer, he helped Thomas Jefferson write the Declaration of Independence.

Franklin was a radical at home as well. He drafted a Pennsylvania Declaration of Rights, in which he argued that the state had a right to discourage people from acquiring large fortunes on the grounds that it would lead to inequality. The assembly rejected this truly radical idea. He also failed to convince the Continental Congress to make proportional representation the basis of the Articles of Confederation. Though only several years earlier Franklin had hoped for reconciliation with Britain, he now argued powerfully for a radically democratic, heavily unified government in independent America. It was a dramatic shift.

Though he lost these battles, Franklin remained a master politician. In 1776, Congress appointed him to a three-person team charged with negotiating for peace with Britain. After these negotiations failed, Congress appointed Franklin, Arthur Lee, and Silas Deane commissioners to France. Their job was to bring France into the war on America's side. Franklin arrived in France on December 3, 1776. In a meeting with the French foreign minister several weeks later, Franklin and the other commissioners succeeded in getting the French to loan money to the struggling American government.

With this first task accomplished, Franklin moved into a house in Passy, a suburb of Paris. He continued to press the French government to help America. In the meantime, he enjoyed himself. In Paris he was a celebrity. John Adams later complained enviously that just about everyone in Paris, from the aristocrats down to the street cleaners, knew and loved Franklin. In April 1778, Franklin joined the Masonic Lodge in Paris, where, to the delight of the French, he and Voltaire publicly declared their friendship.

On October 21, 1778, Congress elected Franklin minister plenipotentiary to France. Before this, he had shared the job of commissioner with Deane and Lee; now he was America's sole representative to France. This new job kept Franklin busy with diplomatic duties, but did not entirely keep him from his other interests. During his hectic schedule he managed to invent bifocal glasses (described in a letter on May 23, 1784) and develop a new way to make lumber last longer by treating it with salt. He also continued his interest in electricity, inventing a new way to test the electrical conductivity of metals. He wrote a number of essays about the aurora borealis (commonly called the Northern Lights). Between his political skill, social celebrity, and intellectual renown, Franklin personally helped to create a powerful friendship between France and America.

Franklin dove into the Revolution with gusto. He was idealistic, radical, and energetic. He passionately wanted America to be independent, strong, and united. He hated war, however, and did all he could to bring peace as soon as possible. Franklin led the effort to negotiate with Britain and never stopped hoping that the two countries could remain friends and allies. For the meantime, however, he wanted to help his new country win. His effort to enlist Canada into the war, if had succeeded, might have won the war for America outright, and might have dramatically changed the course of American history. His ideas for American government, had they been adopted, would have created a much more radically democratic and centralized state. It would have looked much more like the government we have today than the government that was eventually created by the United States Constitution.

Franklin was an undisputed leader of the Revolution. Other patriots relied on him for advice about government, politics, propaganda, and law. George Washington even consulted him for military advice. Franklin was most important, however, as a diplomat. Without France's aid, America could never have won the war. Without Franklin's charm and talent, the French may not have given this aid. The French ministers were impressed by his intelligence and sophistication, his reputation as a scientist and man of letters, and his understanding of British and French politics. He helped convince the French that Americans were serious about winning their independence and would stop at nothing short of it. Recognizing this, the French leaders saw a chance to hurt Britain by helping America.

While the French government respected Franklin, the French people loved him. He was a celebrity. The French delighted in Franklin's jokes and witticisms. French intellectuals, steeped in the philosophy of Jean-Jacques Rousseau, saw Franklin as a sort of noble savage. Rousseau had written about a "state of nature" in which all humans were equal and free. In Rousseau's philosophy, the institutions of society had created inequality and hierarchy. Though Rousseau thought of the "state of nature" as hypothetical rather than historical, many his admirers believed that it was real. They sometimes equated it with America. Dressing and speaking like a backwoods farmer, yet full of wisdom and intelligence, Franklin seemed to be from this "state of nature."

Franklin's popularity was not limited to intellectuals, however. John Adams famously described the Franklin phenomenon. "His name," Adams wrote, "was familiar to government and people, to kings, courtiers, nobility, clergy, and philosophers, as well as plebeians, to such a degree that there was scarcely a peasant or a citizen, a valet de chambre, coachman or footman, a lady's chambermaid or a scullion in a kitchen, who was not familiar with it, and who did not consider him as a friend to human kind."

While nearly everyone loved Franklin in Paris, there was at least one person back in America who hated him: Franklin's own son, William. William had been appointed governor of New Jersey, probably at the request of his father. Franklin had raised William, helped his career, even tended to William's illegitimate son. He had begun his Autobiography as a letter to William, and had hoped William would follow in his footsteps. When the Revolution began, though, William took Britain's side. He remained a loyalist throughout the war, to Franklin's disappointment. After the war started, father and son were enemies; they would never make up.