The 92-year-old queen, whose real birthday is April 21, seemed vigorous as she keenly observed the event.
Meanwhile, a 51-year-old woman was arrested for a public order offence after an item was thrown towards a procession at Trooping the Colour, Scotland Yard said.
After this year's Trooping the Colour, it was time for Meghan Markle, now Duchess of Sussex, to cross another first off her list of royal of events: a balcony appearance at Buckingham Palace.
His proud parents and sister will be among the crowds lining Horse Guards Parade for the ceremony. Kate Middleton, the Duchess of Cambridge, immediately knelt down to comfort her then held her as the parade continued.
But the Queen-and-Meghan road show service a double duty.
Prince Harry appeared to be caring for his wife as they made their first appearance in public since they returned to the United Kingdom from their honeymoon in Canada.
Meghan Markle's entry into the royal family began with her new in-laws appearing to be ever-so-slightly scandalized by her non-traditional wedding day choices and those breaks with the past may have continued this weekend.
The royal pair will open a theatre and science discovery centre after journeying overnight from London on Wednesday. Now, royals like the Queen, Prince Philip, and Duchess Camilla have access to the nine-car vehicle, but Prince Charles uses it the most.
It comes despite her undergoing surgery in early May to have a cataract removed from her eye.
The royal train was first built in 1842 while Queen Victoria was on the throne, as a place for the royal family to stay during business trips, according to People. Aside from Princess Charlotte's fall, she worked her charm by showing off her excellent waving skills.