Trooping the Colour is usually all the sovereign, but this time, King Charles III had to share the spotlight.
She has been taking time away from royal duties, but left Buckingham Palace in a carriage with her children shortly before 11am local time (8pm AEST) local time to watch the parade.
After the ceremony finished, she appeared on the famous balcony at Buckingham Palace alongside other members of the royal family, as a large crowd cheered below.
She confirmed in a statement late on Friday she would attend Saturday's festivities, which are also referred to as the King's Birthday Parade.
A car carrying the Princess of Wales and her children was seen entering the grounds of Buckingham Palace on Saturday morning, ahead of the event, which began at 10:30am local time.
Later, her carriage to Horse Guards Parade followed others carrying members of the royal family, including King Charles and Queen Camilla.
Large crowds lined the Mall, as well as temporary grandstands in Horse Guards Parade, for the event, which began under grey skies and finished amid a downpour.
Kensington Palace has declined to provide specifics on the princess's health, but has described her chemotherapy as "preventative".
In her statement on Friday, Princess Catherine said she was having "good days and bad days" and was "not out of the woods yet".
"My treatment is ongoing and will be for a few more months," she said.
"On the days I feel well enough, it is a joy to engage with school life, spend personal time on the things that give me energy and positivity, as well as starting to do a little work from home."
The fact King Charles was able to participate in the celebrations was a major milestone in his own health battles.
On the same day it was announced Princess Catherine had undergone abdominal surgery, Buckingham Palace announced the king was being treated for an enlarged prostate.
However, it was subsequently revealed he had cancer, and was receiving chemotherapy.
He stepped away from public-facing duties for 12 weeks, but has since returned to some.
Trooping the Colour is an annual parade performed by several military units — collectively referred to as the Household Division — responsible for state ceremonies.
Various members of the royal family who have roles within the units take part, while others watch.
While their functions at Trooping the Colour are ceremonial, all Household Division members are trained combat soldiers, and many have fought for their country.
The Irish Guards, in particular, spent significant time in the past 12 months training Ukrainian troops in Salisbury, Wiltshire.
While the rain largely held off for the duration of the parade, which lasted an hour, as soon as King Charles and Queen Camilla left Horse Guards Parade in a carriage the heavens opened.
It meant all the soldiers were drenched, as were other royals who were in open-air carriages.
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