The queen, a keen horsewoman, has previously taken the salute in person at Horseguards Parade, the site a short ride down The Mall where king Henry VIII once jousted.
But her 73-year-old son and heir Prince Charles stepped in this year, in the latest public sign of his future role as king.
Joining him at the parade of the colour or regimental flag of the 1st Battalion Irish Guards was his sister Princess Anne, 71, and his eldest son, Prince William, 39, both in ceremonial uniform.
Senior royals watching included Charles' younger son, Prince Harry, and his wife Meghan, on a rare visit from California.
But the queen's disgraced second son, Prince Andrew, 62, did not join them.
The queen will make a second appearance on the balcony later to watch a six-minute fly-past of more than 70 military aircraft, including iconic models from World War II.
At nightfall, the queen will be at Windsor Castle, west of London, to take part in a ceremony to light more than 3,000 beacons across the country and the Commonwealth of 54 nations that she heads.
Elizabeth was a 25-year-old princess when she succeeded her father king George VI in 1952, bringing a rare touch of glamour to a battered nation recovering from World War II.
Seventy years on, she is now the only monarch most Britons have ever known, becoming an enduring figurehead through often troubled times.
Britain's first and very likely only Platinum Jubilee will see street parties, pop concerts and parades until Sunday.
It has not yet been confirmed if she will attend a thanksgiving service at St Paul's Cathedral on Friday, while her planned attendance at horseracing showcase The Derby on Saturday is off.
She could yet put in a final appearance -- again from the palace balcony -- on Sunday, at the climax of a huge public pageant involving 6,000 performers.
The queen on Wednesday thanked everyone involved in organising community events in Britain and around the world, and said she was "inspired by the goodwill" shown to her.
"I... hope that the coming days will provide an opportunity to reflect on all that has been achieved during the last 70 years, as we look to the future with confidence and enthusiasm," she added.
The jubilee, held against a backdrop of rising inflation that has left many Britons struggling, is being seen not just as respite for the public after two years hit by Covid, as well as world events.
"All the horrors that's been going on in the world are put behind for us for a day and we can just enjoy really celebrating the queen," said Hillary Matthews, 70.
But the jubilee will also be welcome for the royal family.
Harry, 37, and Meghan, 40, caused shockwaves in early 2020 by moving to North America, from where they have publicly criticised royal life.
In April last year, she lost her husband of 73 years, Prince Philip, and was forced to sit alone at his funeral due to coronavirus restrictions.
Since then, she has struggled with her health, and also the fallout from Andrew's links to the convicted sex offenders Jeffrey Epstein and Ghislaine Maxwell.
Andrew, who in February settled a US civil claim for sexual assault, has effectively been fired from his royal duties.
Attention is increasingly turning to the succession, and the monarchy's future at home and in the 14 other Commonwealth countries where the queen is also head of state.