They ended it thanking the American people for the warm reception they had received.
Three days is a long time on a royal trip.
We have learnt a lot about the Prince and Princess of Wales.
Perhaps the most obvious thing is to see how aligned Prince William and his father King Charles are in the face of controversy and criticism.
The Prince and Princess of Wales's arrival in Boston this week was overshadowed by a row back in the UK.
Lady Susan Hussey's decades of service within the Royal Household did not help when she repeatedly asked a black British charity boss where she was "really" from at a Buckingham Palace reception.
But the King and Prince William were on the same page. She had to go. She went quickly.
What all of this has highlighted is just how tricky the next few weeks will be for the royal family. We have only had the Harry and Meghan trailer so far. Next up will be all six episodes of the documentary series with its promise of personal revelations.
Then after Christmas comes the publication of Prince Harry's memoir "Spare."
It is billed by the publisher as "candid and emotionally powerful" which must send chills down the spines of the Royal Household.
Whoever's truth you believe, wherever your sympathy lies, this will undoubtedly be a difficult time for the Royal Family. It will feel exposed.
Members of the family will have to decide if they want to react to anything that's broadcast or written. Responding comes with risks. Staying silent does too.
Despite the unexpected challenges of this week, the Prince and Princess of Wales leave Boston on an optimistic note.
The Earthshot Prize Awards Ceremony was why they came to this city. It mixed sustainable glamour with environmental ambition.
Three days in Boston has been quite the journey.