Councillor John’s 1,542 vote win gave her a 280 vote majority over Labour candidate Leila Erin-Jenkins, pushing the Greens up to 20 members on Brighton and Hove City Council.
No single party controls the council as Labour has 17 and the Conservatives 13 councillors, plus four sit as independents.
The Green gain leaves councillor Theresa Fowler as the only Labour member left on the council within the Brighton Pavilion parliamentary constituency.
After praising her Green colleagues for their hard work ensuring her election, Councillor John said she wants to follow up on the 'amazing ideas' that came up on the doorstep when she spoke to people during her campaign.
She said: “I want to make sure we start putting in some of the amazing ideas that came up on the doorstep that I’ve been talking to people about.
“There’s low-traffic neighbourhoods, thinking about the way we use our space and our communities. I’m just looking forward to getting back to those things.”
Councillor John decided to stand for election after finding herself 'bored of being cross' as she wanted to put her energy into something positive.
She lives in the ward and is a researcher in human-centred design.
Results of the Hollingdean and Stanmer by-election:
Emma Louise Dawson-Bowling (Conservative Party) 745 votes
Leila Erin-Jenkins (Labour Party) 1262 votes
Nigel Furness (Independent) 24 votes
Alex Hargreaves (Liberal Democrats) 47 votes
Zoë Olivia John (Green Party) 1542 votes
Des Jones (UK Independence Party (UKIP) 35 votes
Rob Somerton-Jones (Trade Unionist and Socialist Coalition) 54 votes
The turnout was 31.9 per cent out of an electorate of 11,758.
There was also a by-election for the Patcham ward.
Results of the Patcham by-election:
Bruno De Oliveira (Labour Party) 879 votes
Charles Daniel Goodhand (UK Independence Party (UKIP) 50 votes
Madelaine Joanna Primrose Hunter-Taylor (Liberal Democrats) 174 votes
Anne Christine Meadows (The Conservative Party) 2011 votes
Eliza Wyatt (Green Party) 1733 votes
The turnout was 43.8 per cent out of an electorate of 11,106.