Ian Conlan, who has served as mayor of Malton for three years, has defected to the Liberal Democrats and announced plans to stand as a candidate in the May 2027 local elections.
The Lib Dems are currently the largest opposition group on the Conservative-led authority.
Currently, the council has 90 councilors and no group has an overall majority.
The Conservatives and Independents group has 44 seats, the Liberal Democrats and Liberals 14, Labor 10, the Greens and Independents eight, North Yorkshire Independents seven, Reform UK five and two councilors are unaffiliated.
Mr Conlan, an environmental campaigner who has been involved in initiatives including the county’s 20s Plenty campaign, said he believed the Liberal Democrats offered the best opportunity to deliver change across North Yorkshire.
He said: “The Lib Dems are the leading opposition on North Yorkshire Council, with a real opportunity to deliver fairer school transport, better active travel policies and more community influence over planning decisions.
“Having led successful local initiatives, including the county’s 20’s Plenty campaign, I know the importance of turning environmental ambition into practical action.
“I believe the Lib Dems will provide the leadership and governance to deliver this change across North Yorkshire.”
Mr Conlan also criticized the leadership of his former party.
He added: “I am delighted to stand as a candidate for next year’s election as part of the Lib Dem campaign to take control of the council.
“I’m frustrated with the direction of the Green Party lately, it’s just not the caring party I joined in 2015. I’m delighted to have found a new home in the Lib Dems.”
Councilor Peter Lacey, leader of the Liberal group on North Yorkshire Council, claimed the party was building momentum ahead of the election.
He said: “We are delighted to welcome Ian to the team ahead of the council elections next year.
“His experience, skills and energy will be a real asset to our growing team, providing fresh perspectives and a strong commitment to local residents.
“I am confident that, working together, we can build on the momentum behind the Liberal Democrats and give voters across North Yorkshire a positive alternative, putting us in a strong position to win in May.”
North Yorkshire Green Party declined to comment on Mr Conlan’s departure.
Earlier this month Cllr Kevin Foster, who represents Hipswell and Colburn on North Yorkshire Council and stood as a Green Party candidate in the 2024 York and North Yorkshire mayoral election, announced that he will now sit as an independent councillor.
His departure follows controversy at the Green Party’s annual conference in Brighton, where members voted to classify Zionism as a form of racism.
The vote divided the party, prompting accusations of anti-Semitism from Jewish organizations and drawing criticism from four of its five MPs.