A 41-gun royal salute and 62-gun salute rang across central London on her big day, which coincides with Easter Monday this year.
In Scotland, gunners from 105th Regiment Royal Artillery fired two 21-gun royal salutes at Edinburgh and Stirling Castles.
The monarch herself was spending the day privately at her second home in Windsor Castle, near London.
The Queen was born on April 21, 1926, and her birthday is also celebrated officially each year in June at the Trooping the Colour service - which this year will falls on June 14.
A portrait of the Queen by renowned British photographer David Bailey was released yesterday in honour her birthday.
The photograph of the monarch, smiling, was taken at Buckingham Palace in March and commissioned on behalf of the UK government's "GREAT Britain" campaign.
The Queen is wearing pearls and a dress by Angela Kelly, who has served as personal assistant and senior dresser to her since 2002.