"The baby weighs 7lbs 3 oz", the Palace revealed. "We just wanted to share this with everybody".
The typewritten note - which was in a gold frame - simply stated: "The Queen and the Royal Family are delighted at the news that Her Royal Highness The Duchess of Sussex was safely delivered of a son at 05.26am today".
The Queen made a special ruling in 2013 to ensure all of Prince William and Catherine's children became royal highnesses, but the dispensation does not apply to other royal babies, including Prince Harry's offspring.
Harry - who is the son of the heir to the British throne Prince Charles and younger brother of Prince William - confirmed the happy news to assembled media and praised his wife for how she coped during the labour.
Given their royal status though, it's highly unlikely that Harry and Meghan will follow any of the United Kingdom rules when it comes to the newborn "Baby Sussex".
The self-confessed big kid was now a father to his own kid - a role Meghan has already said her husband will excel at.
"Just between us, I suspect that Kylie and Shane may possibly make the shortlist", he said in a speech at Australia House in London. I'm so proud of my wife.
"The baby is a little bit overdue so we've had a little bit of time to think about it".
Speaking to the BBC, royal expert and author Katie Nicholl says, "I don't know how much this has caught the palace press office by surprise as well, it's happened very quickly".
He promised more details would be released in the coming days, as the world awaits a first look at the royal baby. - Meghan Markle, the Duchess of Sussex, gave birth on Monday to a "very healthy" boy, Prince Harry announced.
"As every father and parent would ever say, your baby is absolutely fantastic".
Names including Arthur, James and Albert are now hotly tipped by British bookmakers to be chosen by the couple. "How any woman does what they do is beyond comprehension".
Australian Prime Minister Scott Morrison also sent his "huge congratulations" to the couple, tweeting: "Becoming a parent is one of the greatest joys of life!"
After the birth, Queen Elizabeth II, senior members of the royal family, Princess Diana's siblings and Meghan's family were informed.