The Conservatives have held on to their seat in the Hale ward, winning almost half of the votes.
Tory candidate Tom Davey won 2,798 of the Barnet borough by-election vote, beating Alex Brodkin (1,882) into second and the Liberal Democrat candidate Geoffrey Jacobs (487) into third.
The 21-year-old university student, who lives in Langley Park, Mill Hill, will join fellow Conservatives Hugh Rayner and Brian Gordon who already represent the ward.
He said: "I feel pretty happy. It shows that the people in Hale have chosen hope over hate.
"They had three Conservatives and they wanted to continue the service they were getting and I was the right candidate."
He said there was "no one particular issue" he wanted to focus on as a councillor.
But he added: "I think it's just continuing the good service that we've been delivering and trying to improve it where possible.
"One of the things I'm keen on is making sure the introduction of recycling goes in properly."
Councillor Matthew Offord, deputy leader of the council, said that the election of the 21-year-old was the fruit of a deliberate policy to encourage young people to stand for the party.
He said: "We have worked towards having younger people who represent different parts of the community that, perhaps, some of the older people couldn't have done.
"Politics is not just about elderly people, it's not just about middle-aged people. It's also about the future of our communities and we're very pleased that Tom has now joined us in our group."
Mr Offord is the Conservative's prospective parliamentary candidate for Hendon, the constituency currently represented by Labour MP Andrew Dismore, and voters in Hale ward could have a key part to play in the next general election.
"I think the result is a clear indication that people are coming back to the Conservatives having briefly flirted with Labour," Mr Offord said.
"It's a clear indication that in the coming general election, whenever Gordon Brown decides to call it, that the Conservatives will be the leading party and will win in the Hendon constituency."
Andrew Newby, who stood for the Green Party, won 206 votes.
The seat became vacant after Conservative councillor Jane Ellison resigned in March having moved away from the borough.