Saturday's parade, known as "Trooping the Colour", comes just weeks after Charles's coronation on May 6, another ceremony featuring soldiers marching in scarlet coats and bearskin fur hats in central London accompanied to music played by military bands.
By appearing on horseback Charles, 74, revives a tradition which his mother, Queen Elizabeth II, gave up in 1986 when she was 60.
In what is the British military's annual tribute to the monarch, Charles was greeted by a Royal Salute, and carried out an inspection of soldiers in front of 8000 spectators, followed by his son the Prince of Wales, his sister Princess Anne and his brother Edward, the Duke of Edinburgh, also on horseback.
The Queen and the Princess of Wales followed in a carriage.
The colour, or regimental flag, which was trooped in the ceremony belongs to the 1st Battalion Welsh Guards.
Charles then returned to Buckingham Palace with his family to watch a fly-past of around 70 military aircraft and helicopters.
Clear skies meant crowds gathered along the route between the Mall and Horse Guard's Parade saw a full air display, including Typhoon fighters and Red Arrows, jets which were prevented from joining the Coronation fly-past due to low cloud.
The C-130 Hercules also took part, the transport plane's final ceremonial flight before being retired after almost 60 years of service.
Charles became King when Queen Elizabeth died aged 96 in September last year.
Trooping the Colour marks the official Birthday of Britain's monarch and is usually held in June. Charles's actual birthday is November 14.
Dame Anna Wintour. Photo: Getty Images
Vogue's Anna Wintour, author Ian McEwan honoured
Vogue editor-in-chief Dame Anna Wintour, author Ian McEwan and immunologist and geneticist John Bell have been named Companions of Honour, an order limited to 65 people at any time, in King Charles' first birthday honours list.
Wintour, who has been editor of Vogue US since 1988, is one of the most powerful people in the fashion world and a philanthropist who has raised more than $US300 million ($NZ482 million) for the Metropolitan Museum of Art's Costume Institute in New York.
In 2017, Wintour was made a dame by Queen Elizabeth for services to fashion and journalism.
McEwan, who was appointed to the order for services to literature, is the author of acclaimed novels including Amsterdam, Atonement and Enduring Love.
Bell's three-decade-long mission to build vaccines research capability at the University of Oxford, where he is Regius Professor of Medicine, was vital to the development of the Oxford/AstraZeneca Covid-19 vaccine, the government said.
More than 1000 people were rewarded in the King's birthday honours for service in education, healthcare, philanthropy and other areas.
