Mr Farron says he will fight for more affordable housing in Cumbria.
Liberal Democrat candidate Tim Farron has dealt the Conservatives a body blow by defeating shadow education secretary Tim Collins.
With a total of 22,569 votes - a margin of just 267, his victory in Westmorland and Lonsdale, Cumbria, has weakened Michael Howard's front bench.
Mr Farron said the victory was big news and "very life-changing".
He promised to focus on the needs of the constituency and not to play the "Westminster central office game".
He told BBC News: "You don't trust your gut feelings but we always thought that we were just ahead. We knew 500 votes could go either way.
"We fought it like a by-election, on the basis of some very relevant issues, such as local housing.
"My view is that Westminster is work and Westmorland is home. It's all about being a hard working advocate for the people who put me here - somebody who looks after the constituency, as opposed to playing the Westminster central office game."
Affordable housing, sustainable communities, dental services, rural schools, and post offices are the main issues, he said.
He added: "A lot of people thought we were simply planting seeds for the next election, as in many Labour constituencies. But the Tory majorities over the Lib Dems are much lower than Labour's.
"That's not to say that we won't be victorious at the next election. When I joined the party we had only 16 MPs - that's what I call tremendous growth."