The Queen, wearing a black overcoat, appeared on the balcony at the royal Amalienborg residence on Monday afternoon to cheers and Danish flags.
Hundreds of people were present for the occasion, in keeping with Danish tradition on the monarch's birthday.
The Queen's birthday is her first since her husband Prince Henrik passed away earlier this year.
But the late prince was not the only noticeable absence on the balcony.
Crown Prince Frederik, whom newspaper Ekstra Bladet reports to be on a private trip to Greenland, was not present on Monday.
Crown Princess Mary, the Queen's youngest son Prince Joachim and his wife Princess Marie, and the Queen's grandchildren were with her as she greeted the crowd.
The birthday celebration returned to Copenhagen after being held at Marselisborg Palace in Aarhus last year.
Denmark's Royal Life Guards lined up on the square at Amalienborg dressed in their traditional red uniforms.
Crowds at Amalienborg. Photo: Mads Claus Rasmussen/Ritzau Scanpix
Following the public event, the Queen's private birthday celebrations will take place with family and friends at Fredensborg Palace north of Copenhagen.
The occasion marks the first celebratory royal event since Prince Henrik died in February at the age of 83.
A one-month period of official mourning was announced by the palace following the prince's death, but the Queen returned to her duties ten days after the funeral.
Queen Margrethe has been the Danish regent since taking over from her late father King Frederik IX in 1972.