The Princess of Wales, her husband William, their children George, Charlotte and Louis gathered together with King Charles and Queen Camilla to watch a Royal Air Force flypast. They waved and smiled at the crowd, who responded with cheers.
The Princess, also known as Kate, was earlier driven in a state carriage with her children from Buckingham Palace to Horse Guards Parade to watch the celebration, known as Trooping the Colour. She could be seen wearing an Irish Guards regimental broach, a nod towards her role as their honorary colonel.
The family were later seen watching events from a viewing point, while Kate's husband William, the Prince of Wales, rode on horseback.
For Kate, it was her first public appearance since Christmas Day - weeks before her cancer diagnosis.
Kate stepped away from public duties in January, following what Kensington Palace said at the time was surgery for a non-cancerous abdominal condition. In March, amid speculation about her wellbeing, she revealed she had been diagnosed with cancer after the operation, and was undergoing treatment.
King Charles III and Queen Camilla were also taken in a carriage from Buckingham Palace, bringing cheers from the packed crowds as they made their way slowly up The Mall. The King took part riding a carriage rather than on horseback this year due to his own cancer diagnosis.
People of all ages gathered along the length of the Mall for the event. Some onlookers are wearing Union Jack bucket hats and fascinators, while others are waving flags. Some sang the national anthem.
Kate gave an update on her health on Friday, saying she had made "good progress" in her recovery. But she said she expected her treatment to last for a few more months and was "not out of the woods yet."
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Trooping the Colour is the magnificent military spectacle which sees 1,400 officers and soldiers, together with 400 musicians and 200 horses, process through the streets of London from Buckingham Palace to Horse Guard's Parade, while crowds line the route.
The event - one of the highlights of the royal calendar - is a tradition marking the official birthday of the British sovereign for more than 260 years. However, Charles's actual birthday is in November.
Britain's Kings and Queens have doubled up on their festivities since the 18th century, holding both a public celebration - the official birthday - and a more private event on the real date.
The tradition is believed to have started with the party-mad King George II in 1748. Like Charles III, George was born in November, when British weather is often far from ideal.
Proving that point, the weather took a downward turn as events neared their end. Crowds waiting for the King to arrive back at the palace were lashed with heavy rain and gusts of wind. Some onlookers struggled to hold their umbrellas in place.
Speaking about Kate's appearance on the balcony, David Betteridge, 66, told CNN that "she looked well so hopefully that is a good sign."
"It's really nice to see her back actually," he added. "For the most part we stayed dry. But we got pretty wet just after the ceremony when the parade had finished. It rained very hard."
Anna Yang, 30, who was visiting the UK from China on holiday, said she saw the flypast from her hotel nearby. "It was great. It's our first time, even in our own country, seeing all the national aircraft going through the sky," she told CNN. "It was incredible."
But anti-royalists turned out too, some of whom chanted "shame" and "not my king!"
This year's proceedings were adapted as the 75-year-old Charles continues his treatment for an unspecified form of cancer.
Doctors have been encouraged by his progress, allowing him to resume public-facing duties in April, but each engagement he carries out is reviewed and adapted where appropriate.
For the events on Saturday, he carried out his review of troops seated in an Ascot landau carriage alongside Queen Camilla.
Three of the British Army's horses which bolted across in London in April returned to duty and took part in this weekend's birthday parade. The two remaining horses "are enjoying a summer holiday in the Chilterns" and will return to service in due course, according to the army.
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