After the centrist, pro-European party's flop in the European parliament elections, the group split.
Now the Streatham MP has announced he is delighted to join the Lib Dems - a party he once described as untrustworthy.
Mr Davey said: "What he will do when he listens to us and sees the evidence is that the biggest cuts to things like education have happened since 2015 because the Liberal Democrats actually protected schools in the coalition".
"I am convinced the Liberal Democrats, as the spearhead of a broader progressive movement in civil society, offer the best chance to improve the lives of those I represent as well as countless other citizens across our country".
It pledged to push for any Brexit deal negotiated by the government to be voted on at a referendum - or "People's Vote" - in which it would campaign for the United Kingdom to remain in the EU.
Tipped as a possible successor to Miliband, his progress has been curtailed by Jeremy Corbyn, who has shifted the Labour Party further to the left, narrowing the space for Umunna, as a centrist, to make his mark.
In an interview with the Times, when asked if he was prepared for the backlash from his former colleagues, Mr Umunna said: "I've got a pretty thick skin".
Umunna told Friday's Times of London newspaper that he had "vastly underestimated" how hard it is to start a new political party.
I could never ever countenance suggesting voters support Liberal Democrat.
In another previous tweet from 2013, Mr Umunna said: "Vince Cable talks about increasing the minimum wage, but you can't trust a word the Lib Dems say".
LBC presenter Tom Swarbrick spoke to one of the party's leadership candidates Ed Davey and put some of these comments from Mr Umunna to him.
He was widely expected to become the leader of Change UK, but ended up as party spokesman instead.
British MP Chuka Umunna has received a savage response on social media, after revealing he has joined the Liberal Democrats, despite having once declared he "wouldn't be caught dead" wearing the party's rosette.
"There are millions of people, like Chuka, who share those values, and I want them to join our party".