The goalkeeper has ended his 12-year stint with United, the club he supported as a boy, to become Roy Hodgson's third summer signing.
Henderson, 26, impressed away on loan, particularly during two stints with Sheffield United, before getting his chance to challenge David de Gea for the no.1 spot at Old Trafford.
But with just 29 first team appearances to his name, the shot stopper now joins Palace, where he is likely to become first choice, or at least challenge Sam Johnstone and Vicente Guaita for the honour.
But before that, he was keen to pay tribute to what has been his lifelong club, with a social media post that was accompanied by a picture of him in a United shirt as a child.
"Saying goodbye to United is an emotional moment for me," he wrote. "This club has been a part of my life since I was a kid.
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"From growing up as a United fan, to joining the academy aged 14, to stepping out on the pitch at Old Trafford! The journey has been a dream come true and I feel very blessed and fortunate to have fulfilled that dream.
"I want to express my deepest gratitude to everyone in the club who believed in my potential and helped shape my career.
"To everyone in the academy, my teammates, my family, the managers and the back room staff, your guidance and support has helped me become the player and man I am today.
"And to the fans - your support has meant the world to my family and I. I've been United since I was a kid, and being able to wear the United badge on my chest has been an honour beyond words.
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"I never took for granted the responsibility that came with wearing the shirt. We've had some highs and lows but I want to thank all of you for sticking by me throughout.
"Thank you for the memories and I wish the club all the very best for the future."
Further explaining his decision, Henderson told Palace's media: "I've just had a little boy two weeks ago so I just wanted to settle the family down and kick on and find a new home."
United also put out a statement confirming the deal, where they sent their best wishes to Henderson, writing: "Everyone at Manchester United would like to express our pride in watching Dean develop into the model professional he has become and we send our best wishes to him for the future.
"We will be monitoring his progress with an eager eye and hope he enjoys a superb career in the game."