The 96-year-old was met with a standing ovation as she arrived in the castle arena at the Royal Windsor Horse Show.
Celebrities including Tom Cruise, Helen Mirren and Katherine Jenkins appeared in the open air celebration.
It has been running since Thursday night and is the beginning of celebrations to mark the Queen’s 70-year-reign.
The crowd cheered as the smiling Queen arrived and made her way to the royal box, accompanied by the Earl and Countess of Wessex.
She has mobility problems and has had to cancel a number of recent public appearances.
The Queen was spotted on Friday at the horse show, where one of her own horses was announced as a supreme champion.
On Tuesday, she missed the annual reading of the Queen’s Speech for the first time since 1963.
The show, called A Gallop Through History, featured more than 500 horses and 1,000 performers.
Billed as a “personal tribute to our monarchy”, the production takes the audience through more than five centuries of history.
It features international and national military, equestrian and dance displays.
Tom Cruise announced the performance of the King’s Troop Royal Horse Artillery as “one of the most enthralling, thrilling, heart-in-mouth displays”.
The Trinidad and Tobago Defence Force’s steel drum troupe performed a rhythmic version of Abba’s Dancing Queen – which the Queen appeared to enjoy.
She was also seen clapping during a traditional display from the Royal Cavalry Oman, a show of horsemanship that saw horses lying down on command.
The Queen’s Platinum Jubilee 2022 – timetable of key events
Thursday 2 June – Queen’s birthday parade and Trooping the Colour takes place – as Platinum Jubilee Beacons across in 2,000 towns and cities light up
Friday 3 June – National Service of Thanksgiving is held at St Pauls. Ringing of Great Paul, the restored biggest church bell in the country
Saturday 4 June – The Derby at Epsom Downs Racecourse – and BBC Platinum Party at the Palace
Sunday 5 June – Big Jubilee Lunch and street parties. Platinum Jubilee Pageant, a procession and performance in central London, involving 10,000 people and the Gold State Coach