George III became king of Great Britain and Ireland in 1760 and sought an active role in choosing ministers and shaping policy. The end of the Seven Years' War brought extensive imperial gains but also debt and disputes over how colonies should contribute. His early reign involved unstable ministries, controversy over Lord Bute, and conflict with the political agitation associated with John Wilkes. He worked within parliamentary government while attempting to maintain a meaningful royal influence.

During the American crisis, George supported ministers seeking to assert parliamentary authority and opposed the colonists' armed resistance. He endorsed continued prosecution of the Revolutionary War under Lord North. Military failure, including Yorktown, and parliamentary opposition eventually forced a change of government and acceptance of American independence. Although the Declaration portrayed him as the central author of colonial grievances, imperial decisions involved ministers, Parliament, officials, and competing political interests rather than the king acting alone.

After further ministerial struggles, George appointed William Pitt the Younger in 1783 and supported the administration that followed. The king's influence remained important in patronage and major constitutional questions. His opposition to Catholic emancipation contributed to Pitt's resignation in 1801, after the union creating the United Kingdom of Great Britain and Ireland. The French Revolution and wars with France occupied much of the later reign, with successive governments mobilizing resources against revolutionary and Napoleonic power. George also supported the Royal Academy and cultivated interests in science, agriculture, and the royal collections.

Episodes of severe illness interrupted his ability to govern, notably in 1788–1789. The precise medical explanation remains debated, and historical diagnoses should not be presented as settled fact. After renewed and prolonged incapacity, his son became prince regent in 1811. George remained monarch in name until his death in 1820 while the regency government managed the final defeat of Napoleon and postwar challenges. His long reign encompassed both the loss of Britain's American colonies and the expansion of British power elsewhere. Assessments of his political conduct distinguish personal conviction and constitutional influence from the stronger claim that he exercised unrestricted absolute rule.