The Greens have become the joint second largest party in Brighton and Hove City Council after winning the Goldsmid by-election.
The election of Alexandra Phillips gives the party 13 seats, bringing them level with Labour.
The win will continue to instill the Greens with the belief that they are a party on the rise both locally and nationally.
The results were announced just before midnight on Thursday night.
Miss Phillips won with 1,456 votes, beating Conservative candidate Andrew Wealls, who received 1,104 votes, into second place.
The Labour candidate Lis Telcs came third with 816 votes, followed by Liberal Democrat's Howard Spencer with 280 votes and UKIP's Maria McCallum with 129 votes.
Miss Phillips, 24, pledged to make the issue of school places her top priority and has called for two new primary schools and a new secondary in the city.
She is the only candidate to live in the Goldsmid ward that stretches around St Ann's Well Gardens, the county cricket ground and Hove station.
Miss Phillips said: "I am really excited about the changes that we can make.
"It is our first seat in Hove and the first seat taken from the Tories. I think that this builds on the momentum we created in the Euro elections. It is massive news.
"Hopefully this means we can put pressure on the Tories and concentrate on our policies of a 20mph maximum speed limit in Brighton and Hove and creating a living wage, which is more than the current minimum wage."
The by-election, which saw a 32.85% turnout, was billed as the poll for the city's “king-maker”, as the winner could determine who has control of the council.
It was sparked by the resignation of “missing” Conservative councillor Paul Lainchbury.
Mr Lainchbury resigned after he came under fire when it was revealed he had not attended committee meetings regularly for some time.
The Green victory has enhanced the possibility of a coalition between themselves and Labour, which could enable them to wrestle control of the council from the Conservatives.
However sources have said that an official coalition is unlikely after a fierce campaign between the two parties but it is thought that they will work together to increase pressure on the Tories.
Councillor Gill Mitchell, leader of the Labour group, said it was too early to say whether the parties could form a coalition but said that the Green victory had hurt the Conservative party.
She said: "The Conservatives are fatally wounded and they will not be able to get anything through the council.
"This will force them, at the very least, to work in a more consensual manner. I am determined to work with the other parties and ensure stability."
Ayas Fallon-Khan, deputy leader Brighton and Hove City Councillor and Goldsmid councillor, admitted that the Conservatives were disappointed at the result but denied that they were "fatally wounded".
He said: "It is a disappointment. It was a particularly crucial election but Goldsmid is a very transient ward.
"We still have almost twice as many councillors as they do and I think it unlikely that they will form a coalition that could work.
"I think it is much more of a wound for the Labour party and something that they have been dreading. The Greens are now catching them up."
It is the first time the Greens have taken a seat from a Conservative candidate in Brighton and Hove City Council elections.