The pair visited Cheshire together in June two years ago, where they officially opened Mersey Gateway Bridge and visited Chester's Storyhouse Theatre.
Together, the Queen and Meghan also attended a lunch at Chester Town Hall as guests of the city council.
The post was followed shortly afterwards by posts from Kate and William's Kensington Palace account and the Clarence House account, run by the household staff of Prince Charles and the Duchess of Cornwall.
In the wake of release of Prince Harry and Meghan's controversial biography Finding Freedom, which claims to lay bare a rift between the couple and the rest of the Royal Family, the messages from the Queen, Prince William and Kate, and Prince Charles and Camilla is thought to be a universal show of support.
The Queen did not post on Meghan's birthday last year.
The Duke and Duchess of Sussex deny giving any interviews or contributing to the book directly, but the intimate nature of some details raises questions over who the sources were.
The explosive biography has claimed Prince Harry and Meghan "hurt" the Queen by unilaterally mapping out their future royal roles without her approval. Her Majesty is said to have been "blindsided" by the launch of the SussexRoyal website which accompanied their Megxit announcement in January.
Prince Harry, 35, and Meghan moved to the US with their 14-month-old son Archie just a few weeks before the coronavirus pandemic shut down the globe, and like everyone else have been living under lockdown.
The family are living in a stunning mansion owned by filmmaker Tyler Perry.
Birthday celebrations are likely to be low-key as the state of California they now call home reportedly has the highest number of Covid-19 cases in the US.
