Jon Tandy, who was Shrewsbury mayor in 2013/14, and served as a Labour councillor in the town from 2003 to 2017, said he had taken the decision after losing confidence in the party.
Mr Tandy, who is no longer a councillor, but stood as a Labour candidate to contest the Shrewsbury & Atcham seat at the 2010 general election, said: "I was a Labour councillor for 16 years, and served Shrewsbury as mayor.
"I even ran to be our Labour MP. Joining another party is not a decision I take lightly, and has played on my mind for some time.
“Ultimately, the Labour Party I joined 35 years ago is gone. The influence of Corbyn and Momentum is still present like a nasty hangover, especially in Shrewsbury and Atcham. I can’t sit by and stick with them when there are other strong alternatives locally.
“I’m born and bred in Shrewsbury, I love this town. The Liberal Democrats are providing the community leadership that we need to keep making Shrewsbury a better place to live for everyone.
"They really care about our town, and I’m going to be backing the Lib Dems all the way to win votes and seats from both the Tories and Labour.”
Lib Dem Councillor for Bowbrook, Alex Wagner said: “Having someone with Jon’s experience and talent decide to join the Liberal Democrats is a huge boost to us locally.
"It is a real honour to welcome him to the party, and I cannot wait to see what he has in store next.
“Jon’s campaigning record is second to none, and I know he’s already getting stuck in, working hard and holding local leadership to account in his local community in Harlescott.”
Responding to the announcement, Paul Forrest, chair of Shrewsbury & Atcham Labour Party, rejected the criticisms and said they showed Mr Tandy had not attended a meeting for some time.
He said: "I first came to live in Shrewsbury two or three years ago and have never encountered Mr Tandy at any of our meetings."
He added: "The comments about being a Momentum-led PLP are laughable.
"Our last leaflet had Rachel Reeves on the front talking about the cost of living crisis. We had Pat McFadden visit the constituency to talk about the Starmer-ite approach to the economy.
"Obviously he has not been to a meeting for so long he does not know how committed we are to winning the next election."