Conservative politician Craig Mackinlay has been cleared of knowingly falsifying election expenses.
The MP for South Thanet had denied making false declarations during his 2015 campaign, when he beat then-UKIP leader Nigel Farage to the seat.
Mr Mackinlay, 52, had been accused of failing to declare more than £60,000 spent on staffing, hotels and adverts.
Tory party employee Marion Little, 63, was given a suspended sentence after being found guilty of two charges.
Little, who had effectively run the 2015 election campaign, was found guilty of two counts of encouraging or assisting an offence, but cleared of a third.
The long-time Tory party member, from Ware, Hertfordshire, had "acted dishonestly by preparing returns she knew were not completed nor accurate," Judge Mr Justice Edis said.
He described Little as being "a senior and respected employee of the Conservative Party at its Central Headquarters and has been so for many years", and said her actions were a "sustained and deliberate course of conduct".
'Author of falsehood'
The judge said Little had presented falsified documents to Mr Mackinlay and Mr Gray for signing which "they did so in good faith not knowing what she had done".
"No-one can know whether her misconduct had any effect on the outcome of that election but she plainly intended that it would.
"She was the author and origin of this falsehood."
Handing her a nine-month sentence, suspended for two years, the judge said Little had been "carried away by her conviction" that defeating Mr Farage was an "overwhelmingly important political objective".
He said the only reason she was not being given a custodial sentence was because she was caring for her husband, who is terminally ill.
Jurors at Southwark Crown Court had deliberated for more than 53 hours before reaching their verdicts.
Mr Mackinlay responded on Twitter, simply writing #notguilty.
Prosecutor Michelle Nelson told jurors South Thanet would usually have been seen as a safe seat, but Mr Farage's run as UKIP candidate had been regarded as the chance to "knock him out" of politics.
Mr Mackinlay told the court the Conservative Party HQ had "recognised the fear of Farage" and offered to pay for a campaign manager.
He said the South Thanet seat would be "looked at and pored over very carefully" and said he was told during the 2015 campaign that spending was "perilously close" to the limit.
The prosecution's case centred around claims some costs for activists and party workers were recorded as national rather than local election expenses, to ensure spending limits weren't breached.
Declared spending came in under the £52,000 constituency limit, but prosecutors claimed more than £60,000 went undeclared.
Mr Mackinlay eventually won the seat, in what the court previously heard was a "nasty" election.
The MP's agent, Nathan Gray, was acquitted on 13 December.
Mr Gray, 29, of Hawkhurst in Kent had faced one count of making a false election expenses declaration.
Mr Mackinlay said: "It's been nearly three years of pure hell. This has been a really tough time.
"There are questions that need to be asked and the Electoral Commission needs to get together with political parties to make sure that never again a candidate or an agent can be accused of these things."
By Helen Catt, political editor, BBC News South East
This trial has given a rare glimpse behind the rosettes, the posters and the doorstep handshakes into the party machines behind election campaigns.
In this case, it's not been a pleasant one.
A tale of a "win at any cost" mentality that led to spending laws being deliberately breached and then hidden by a party official from the Conservatives' own candidate and his election agent.
Craig Mackinlay described himself as a "small cog in a big gearbox" but it was he who ended up seeing his job, his reputation and even his freedom in jeopardy.
That has made other MPs nervous.
The 2015 election in South Thanet was held under a unique set of circumstances but could the fallout force a rethink of the way election campaigns are conducted in future?
A spokesman for the Conservative Party said while being "disappointed" with Little's conviction the case had "highlighted just how unclear and fragmented electoral law was in 2015".
He said: "There is clearly a need for greater clarity and guidance on the law. We will take further legal advice and continue to meet with the Electoral Commission to seek this."
Mr Farage said: "This verdict shows that there are no election rules for the big parties in British politics, they can wilfully overspend without any consequences."
The Election Commission has welcomed Little's conviction.
A spokesman said: "It is vital that offences under electoral law are properly investigated, including by the police when appropriate.
"This outcome should serve as a deterrent to anyone who would seek to circumvent the proper reporting of campaign spending."
He said the commission was continuing to call for for electoral law to be reviewed.
Correction 1 February 2019: We previously reported Craig Mackinlay was cleared of "falsifying election expenses". We have changed this to "knowingly falsifying election expenses". We also previously said Marion Little was a "party activist", however, the judge described her as "a senior and respected employee of the Conservative Party", so we have amended our story to reflect that.