Sandra Gidley, the Liberal Democrat MP for Romsey in Hampshire for the past 10 years, has lost her seat to the Conservatives.
Mrs Gidley was first elected in a by-election in 2000 after Romsey's Conservative MP Michael Colvin died in a fire at his home in Tangley.
She held on to the seat in 2001 and 2005 but saw her majority plummet to just 125 votes in 2005.
Mrs Gidley said the Tory campaign had been "negative and personal".
Boundary changes had meant the constituency changed to include Romsey and Southampton North.
Caroline Nokes won the seat with 24,345 votes ahead of Mrs Gidley, who polled 20,189.
Meanwhile, in her neighbouring constituency of Eastleigh, Liberal Democrat Chris Huhne retained his seat, increasing his share from 38.3% in 2005 to 46.5%.
In a statement, Mrs Gidley said: "I am obviously disappointed that I will no longer be representing Romsey and Southampton North, but take heart from the fact that voters of all parties told me that they appreciated the work I have done over the past 10 years.
"Of course, I would like to have been a part of a Parliament facing difficult and challenging times, but I will continue to do all I can to secure the Liberal Democrats' objective of a fairer Britain.
"Despite my personal disappointment, I am confident that the Liberal Democrats will do well in this election.
"Unfortunately, the Conservative campaign has been negative and personal, and it only demonstrates that the Tory party, at heart, has not changed at all.
"However, I wish the people of Romsey and Southampton North all the very best, and wish them well with their new MP."