Tessa Munt received 24,560 votes to take the Wells seat
The Liberal Democrats have taken the seat for Wells, previously held by the Conservatives for 55 years.
Tessa Munt beat David Heathcoat-Amory, the MP since 1983, by exactly 800 votes.
Elsewhere in Somerset the Lib Dems held on to four of their seats and the Conservatives held three.
Labour's Dan Norris lost his bid to reclaim the former Wansdyke constituency, now North East Somerset, to the Tories' Jacob Rees-Mogg.
Ms Munt received 24,560 votes compared with 23,760 votes for Mr Heathcoat-Amory, who had to pay back almost £30,000 following the House of Commons expenses audit.
Labour's Wells candidate, Andy Merryfield, had 4,198 votes.
Mr Heathcoat-Amory said: "Expenses damaged all incumbents and perhaps me particularly. I faced a negative campaign, that's the tough way politics is.
"I've enjoyed every minute of representing this constituency for the last 27 years and I leave, I hope, with a constituency in good shape."
For the Tories, Liam Fox held North Somerset. Ian Liddell-Grainger was re-elected in Bridgwater and Somerset West and John Penrose held Weston.
David Heath held Somerton and Frome, Jeremy Browne held Taunton Deane, David Laws held Yeovil and Don Foster, Bath.
Mr Rees Mogg received 21,130 votes to take the North East Somerset seat from Mr Norris who had 16,216 votes.