Martin Swinbank was elected alongside long-serving Gordon Castle (Conservative) in the two seats Alnwick has at County Hall.
He takes the seat vacated by Robbie Moore (Conservative) who resigned after being elected as MP for Keighley in West Yorkshire 2019.
It means the Greens will have two councillors making up the new-look county council after Nick Morphet also took the Humshaugh seat from the Tories.
Cllr Swinbank said: “I’m delighted that we have two Greens on the county council for the first time.
"There are local issues such as the public toilets and bus station where improvements definitely need to happen but we also need to look after our green spaces and enhance them, promoting sustainable travel such as cycling and walking and bus services.”
Cllr Swinbank, who is a member of the Alnwick branch of Friends of the Earth, added: “We really need to bring down carbon emissions, to try to get green jobs locally and to make sure everyone has a safe, warm and climate friendly home.
“We need to get everyone on board with this climate change journey and that means working across parties. This affects all of us – it’s the future of our children and grand-children.”
Cllr Swinback, who has been a town councillor for eight years, added: “People have a much greater awareness of the issue and the need to do something. Those I’ve spoken to during the campaign are well aware that we need to take this more seriously.”
Meanwhile, Cllr Castle retains the seat he has held since 2005.
He said: “I am delighted to get in and look forward to working constructively for Alnwick. I’m very grateful to those who continue to support me and will work hard for those who didn’t.
"I am sorry my running mate (fellow Tory candidate Reuben Carr) didn’t make it but the Greens ran a very strong campaign.
"Martin Swinbank and I know each other very well and I expect us to work well together going forward.
"I take representing Alnwick very seriously but I also enjoy it.”
He was also pleased with the high turnout of 76.8%
"It shows that local government is important and people here recognise that,” he said.