Three parties in as many months....who's next?
Both left the Conservative Party in March to form The Independent Group for Change - known as Change UK - alongside eight former Labour MPs, and former Conservative MP Anna Soubry.
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.
Following disappointing election results, where Change UK failed to make major gains but the Lib Dems came second after the Brexit Party, Mr Umunna went independent.
"I've seen his record over the years and he's basically a liberal".
A Lib Dem spokesperson said "We've had conversations with a number of MPs before and since Change UK happened".
"I know that he will be a great asset to our party not just on Brexit, but in fighting for the liberal and social democratic values that we share".
"He joins alongside 20,000 people across the country just this month, demonstrating clearly that the Liberal Democrats are the biggest, clearest and most formidable force in the liberal centre-ground of British politics today".
When asked by the Times if he was prepared for a backlash from his former colleagues, Mr Umunna said: "I've got a pretty thick skin". "And I'm not sure what more I could do to prove that I'm not, not a careerist".
Umunna said in comments reported by the BBC that he had been wrong to think "millions of politically homeless people... wanted a new party".
Mr Umunna's defection was also welcomed by the two candidates for the Lib Dem leadership.
He was widely expected to become the leader of Change UK, but ended up as party spokesman instead.
But he said "things have changed" since then and the Lib Dems are now an "anti-austerity" party.
Tom began by citing a quote from 2017 where Mr Umunna said: "The Liberal Democrats are trying to bury their recent past as the enablers of Tory austerity, but working people will not forget or forgive the damage they did in government and what it is still doing to our communities".