Councillor Richard Jones was not chosen by his former party for the Old Warley ward, where he has represented since 2019, amid wider claims of a "major cull".
He is one of 17 Labour councillors on Sandwell Council who has either stood down or been deselected ahead of voters returning to the polling booth on May 4.
But now it has been confirmed Councillor Jones, the first openly gay mayor of the borough, has now joined the Lib Dems – and will contest the next election.
Speaking in an announcement video, he said: "I'm really pleased I've joined the Liberal democrats for Old Warley and to be your candidate. I've been a councillor in Old Warley since 2019 and have really seen some changes throughout that time.
"I want to carry on with those changes and really work for the local people and I think the Liberal Democrats give me the opportunities to do that. So I'm really pleased that I can be the candidate for the local elections in May and hopefully I can rely on your vote."
The decision means Councillor Jones, who said he was "absolutely gutted" by the news of his deselection, is now the only Lib Dem representative on the largely Labour-dominated authority.
Meanwhile for Labour, of the 24 candidates selected for the May elections 17 are new after nine sitting councillors were canned. The decisions were made following an interview process involving national and regional Labour figures and the Labour group Local Government Association. Eight more have decided to stand down themselves.
Those on the way out include two other members of the current mayor's family – brother Stephen (Princes End) and mother Olwen (Wednesbury South) – as well as his partner Richard McVittie (Newton).
Mark Smith, chair of the Sandwell Liberal Democrats, said it was positive to have a councillor for the party back on Sandwell Council who can "hold our Labour administration to account" – as he praised Councillor Jones' positive work as mayor.
It is believed Chipiliro Kalebe-Nyamongo has been selected to fight the Old Warley seat for Labour come May. A full list of candidates will be revealed closer to the time.