Galloway was born in Maryland in 1731 before moving with his family to Pennsylvania in 1749. He studied law, was admitted to the bar, and began to practice in Philadelphia. In 1753 he married Grace Growden, the daughter of a wealthy landowner and colonial Supreme Court justice. The two had four children, but only their daughter Elizabeth survived to adulthood.
Galloway was elected to the Pennsylvania Assembly in 1756 and served as Speaker of the House from 1766 to 1774. He and Benjamin Franklin were political allies in the fight to remove Pennsylvania from the control of the Penn family and turn it into a Crown colony. During the American crisis, Galloway opposed the Stamp Act and the Townsend Duties, but he believed that Parliament had the right to tax the American colonies.
In 1774 the Pennsylvania Assembly chose Galloway as a delegate to the First Continental Congress along with John Dickinson. Galloway became the leader of the moderate faction that wished to remain united with Great Britain, and on 28 September, he introduced the Galloway Plan of Proposed Union, a streamlined version of Franklin's Albany Plan of Union, proposing to create a united colonial government under the British crown. Dickinson persuaded Galloway not to offer the plan for a vote; instead, the Congress voted by a one vote majority to table the plan. On 22 October, as the Congress was winding down, Galloway tried to persuade the Congress to take up his plan again, but was unsuccessful.
After the Congress adjourned, Galloway began to claim that his plan had been rejected by a single vote, a claim that Sobel repeats. When news of Galloway's claim reached London, members of the British government cited it as proof that the colonists were already set on independence. When the Second Continental Congress met in May 1775, Galloway declined membership, and resigned as Speaker of the House of the Pennsylvania Assembly.
After General William Howe captured New York City, Galloway moved there, and became a leading figure in the city's Loyalist community. When Howe set sail for Chesapeake Bay in his campaign to capture Philadelphia, Galloway accompanied him; after the fall of Philadelphia in September 1777, Galloway became the head of the city's civil administration under Howe.
General John Burgoyne's victory at the Battle of Saratoga and capture of Albany, New York in October 1777 allowed moderate conciliationists to gain control of the Pennsylvania Assembly. They selected Dickinson as a delegate to the Continental Congress, and he persuaded Galloway to join him in January 1778. As Congress entered negotiations with the Earl of Carlisle in May for an armistice, Galloway was chosen as President of the Congress, and he and Carlisle reached an agreement on 27 May which the Congress ratified on 12 June 1778, ending the North American Rebellion.
Galloway and Dickinson were both invited to London in 1780 to help Lord North draft a new instrument of government for the North American colonies, known as the Britannic Design, which relied heavily on Galloway's plan of union. Following passage of the Britannic Design in January 1781, Dickinson and Galloway returned to North America, the former to become the first Governor-General of the Northern Confederation, the latter to resume his place in the Pennsylvania Assembly.
The Galloway family became a notable one in the Confederation of North America, particularly Galloway's great-great-great-great grandson Owen Galloway, the President of North American Motors and creator of the Galloway Plan of 1922. Since Galloway had no surviving sons by his wife Grace, he must have remarried after her death and had one or more sons by his second wife.
Sobel's sources for Joseph Galloway's career during the North American Rebellion are James Elson's Dickinson and Galloway in the Crisis Years (New York, 1901) and Eric Bjornson's The Failure of the Middle: The Triumph of Radicalism in America in 1774 (London, 1965).