Per google:
As of 2026, the following monarchs have made official visits while on the throne:
King George VI (1939): The first reigning British monarch to visit. He and Queen Elizabeth (later the Queen Mother) toured Washington, D.C., New York City, and attended a famous hot dog picnic with President Franklin D. Roosevelt at Hyde Park.
Queen Elizabeth II (1957, 1976, 1983, 1991, 2007, 2010): The most frequent royal visitor, making five state visits and one official visit to the United Nations. Key moments included celebrating the U.S. Bicentennial in 1976 and being the first British monarch to address a joint session of Congress in 1991.