Here is a selection of what various Labour big wigs, political commentators and media figures have made of Miliband’s decision and his parliamentary career. And we're interested to hear your thoughts on Miliband's career and departure. Please leave a comment in the box below.
Ed Miliband: Having spoken to him a lot over the past few months, I know how long and hard he thought about this before deciding to take up the offer. I also know how enthusiastic he is about the potential this job provides... As for us, we went through a difficult leadership contest but time has helped to heal that. I will miss him. But although he is moving to America, I know he will always be there to offer support and advice when I need it... British politics will be a poorer place without David. But his huge talents will be serving people around the world. I hope and believe that at some point in the future he can once again make a contribution to British public life.
Tony Blair: I congratulate David on his appointment to a major international position. It shows the huge regard in which David is held worldwide. I'm sure he will do a great job. He is obviously a massive loss to UK politics. He was the head of my policy unit and then a truly distinguished Minister in the Government and remains one of the most capable progressive thinkers and leaders globally. I hope and believe this is time out not time over.
Tessa Jowell: He leaves optimistic about the united force Labour has become under his brother & a conviction that his brother will be a great PM.
James Forsyth: I must admit that I have always thought that David Miliband wasn’t as good a politician as his admirers made him out to be. But if he had gone for it in the summer of 2008 or even 2009, I expect he could have toppled Gordon Brown and become Prime Minister. The real failure of his political career was not losing to his brother, but not following James Purnell out of the Cabinet in June 2009.
John Rentoul: [David Miliband] could possibly have been shadow chancellor when his brother was elected leader, but couldn’t face it. Now, even if he could face it, he knows it is not going to be offered. Ed M has reconciled himself to fighting the election with Balls at his side: difficult as their relationship is, he gets to be leader – something to which Balls is equally reconciled. If David M cannot be leader, he should be shadow chancellor, but it is a measure of Labour’s problem that he won’t be. The other part of his calculation (I guess) is that he wouldn’t ever be leader either. If anything happens to Ed M before the election, Yvette Cooper would win the leadership contest. And it looks quite possible that Ed M might be prime minister after the election. Even if Labour lost, David M’s chances in the subsequent leadership contest would not be great.
Rafael Behr: There are quite a few people inside Labour who will be bitterly disappointed at David’s departure. One inevitable interpretation is that it cements the victory of the old Gordon Brown faction over the forces of Blairism. That is plainly the gloss that Conservatives will gleefully apply. It is an interpretation that carries some resonance for the generation that carries scars from New Labour's epic vendetta.
Nick Watt: There is a growing feeling that Labour could win the next election, though supporters of Tony Blair say Labour needs to be much further ahead of the Tories for that to happen. "The whole thing is very depressing," one senior Labour figure said. "It just feels as if there is no future for David's kind of politics in the Labour party. This is bad news."
David Aaronovitch: The tragedy for David Miliband was that he was beaten for the leadership by the one man he couldn't criticise or differ from.
Damian Thompson: He reminds me of Michael Heseltine: boy, does he take a good photograph but he lacked – or withheld – that ounce of charm that could have tipped the leadership election his way. And, I'm convinced, the general election too. Ed is, well, a numpty, and no finely rehearsed speeches will change that. His older brother isn't, but when he was foreign secretary he swept past Labour colleagues like a renaissance prince. It's why he never wore the crown.