The King bore an uncanny resemblance to his father, the late Duke of Edinburgh, as he rode down The Mall in a crisp red tunic and towering bearskin for Trooping the Colour.
Sitting erect in the saddle, His Majesty became the first monarch to take part in the military ceremony on horseback for 36 years.
However, he was required to draw on his extensive riding experience to nurse his mount through the occasion, and at one stage a groom had to step forward to keep her calm.
The new commander-in-chief of the Armed Forces rode out from Buckingham Palace shortly before 11am on a gleaming black mare, Noble, followed by the Prince of Wales, Duke of Edinburgh and the Princess Royal.
The seven-year-old horse, a gift from the Royal Canadian Mounted Police, had done extensive training for the occasion, with the general in charge of the parade reporting that she had not put a foot wrong.
However, processing down The Mall, which was lined with thousands of well-wishers, the horse appeared visibly distracted and, at times, unsettled.
The King could be seen constantly mouthing words to her in an effort to keep her focused.
He and the other senior Royals rode in the midst of a sovereign’s mounted escort, formed by the Life Guards and Blues and Royals.
As the national anthem was played on Horse Guards Parade, a member of staff was required to help settle the horse.
The last time the sovereign took the royal salute at the parade when mounted was in 1986, when Queen Elizabeth II took part for the final time on Burmese, also a gift from the Mounties, whom she had ridden at the ceremony for 18 years.
It was a day laden with symbolism, and with weather that the organisers of the Coronation could only have dreamed of.
The first sovereign’s Birthday Parade of the new Carolean era, Saturday marked the first time all five regiments of Foot Guards have paraded together since 1989.
The Welsh Guards took centre stage among the roughly 1,500 soldiers taking part, parading their regimental colour, replete with battle honours, before His Majesty.
The colour was trooped by the Ensign, 23-year-old Second Lieutenant Freddie Hopkinson, who described it as “the absolute pinnacle of my life”.
The King, who was the longstanding Colonel of the Welsh Guards, wore the tunic of the Guard of Honour Order, the Welsh Guards, in recognition of the regiment’s leading role on the day.
The music had a distinctly Welsh theme, with Bread of Heaven and Men of Harlech among the numbers played by the mass bands. Welsh Diadem, a composition by Lance Corporal Gareth Trott, a Welsh Guards musician, was also played.
The King also wore the cypher of his late mother.
His own cypher – CR – was spelt out in spectacular effect by 18 Typhoon fighter jets who flew in close formation to form the letters.
It was part of a military flypast including Second World War Hurricanes, a Spitfire and a Lancaster bomber from the Battle of Britain Memorial Flight.
The flypast, watched by the Royal family from the balcony of Buckingham Palace, finished with the Red Arrows trailing red, white and blue smoke.
The King’s participation as monarch was a lifetime in the preparation, with His Majesty first attending Trooping the Colour as a three-year-old in 1951, riding in a carriage with his grandmother, Queen Elizabeth the Queen Mother, and aunt, Princess Margaret.
He first took part on horseback in 1975.
Saturday’s ceremony was also the first where the mounted cavalry was led by a mare, the drum shire horse, who was formally renamed Juno by the Queen at Clarence House last week.
Rishi Sunak, the Prime Minister, and his wife Akshata Murty were among the dignitaries looking on.