Harry and his wife, Megan Markle, were attending a charity performance of the hit musical "Hamilton".
Afterwards the duke joined the cast on stage and broke into mock-song, briefly assuming the role of King George III, his sixth great-grandfather.
During the evening, the Duke of Sussex joked about playing his ancestor King George III in Hamilton, the hip-hop musical that mocks the British monarchy during the founding of modern-day America.
'You say -' Harry sang only for the crowd to cheer so hard he stopped.
Lin-Manuel Miranda, Hamilton creator and star, pointed out the significance of Harry's choice of song.
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Prince Harry stole the show at London's West End after bursting into song at the hit United States hip-hop musical Hamilton.
"I tried to be calm", recalled Rachelle Ann Go as she shared a photo of her standing within arm's reach of Prince Harry.
Meghan took things up a notch as she donned a daring yet stunning Judith and Charles black tuxedo mini-dress, breaking the royal protocol of wearing dresses below the knees, for the event.
Joining Harry on stage afterwards, Mr Miranda said: "Smash cut to 2018 I'm sitting next to his sixth-great grandson".
According to the Post, Prince Harry reportedly thanked Miranda adding, "Honestly, this is a huge deal to us and it's going to be a fun night". "Ladies and gentlemen, the Duke of Sussex".
It is the third time she has seen the award-winning production.