Trouble broke out in June 2014 when Bowden issued the couple with notice to quit Mellinzeath, which is set in about nine-and-a-half acres of land.
The Smyth-Tyrrells took the dispute to the High Court in London, insisting they had the right to stay.
The couple, whose home Bosvathick is about three miles away, had poured cash into making Mellinzeath a draw for holidaymakers, the court heard.
Work they carried out on the cottage and barn had increased the property's value from, at most, £190,000 to £375,000.
Following floods in 2012, they had dredged the millpond, re-ordered and banked the stream, also altering its course.
And, despite having rented out Mellinzeath for tourists, the couple said they have yet to recoup their investment.
But after a hearing at London's High Court, Judge Paul Matthews has ruled that Mr and Mrs Smyth-Tyrrell have to leave.
The cottage is part of the Bowden family's 750-acre Boswidjack Farm, in Constantine and was first rented to Mr Smyth-Tyrrell in 1993 for just £400 a year.
The original lease was for 15 years, but the Smyth-Tyrrells stayed on after that on a yearly tenancy, paying an increased rent of £1,000 a year.
Ruling against the couple, Judge Matthews said there had never been a "common understanding" that they could stay on indefinitely.
Neither Bowden nor his father had given Mr Smyth-Tyrrell any "promise or assurance" that he could stay beyond the initial 15-year term.
The Smyth-Tyrrells never had any long-term security of tenure and money they spent on Mellinzeath was "at their own risk", the judge ruled.
He concluded: "I find that Mr and Mrs Smyth-Tyrrell here played a long game to try and get the ownership of the land from their neighbour without letting on that this was what they were trying to do.
"In the meantime, they thought they could recoup their expenditure - and indeed make a profit - from the refurbishment of the property and then letting it for holiday lets.
"If it has not succeeded it is because it was a speculation which could have gone either way but which in the event has not proved as profitable as hoped.
"If they had a made a fortune, recouping their expenditure many times over, Bowden could not have complained.
"Mr and Mrs Smyth-Tyrrell have had what they bargained for, and indeed more.. Enough is enough."
The judge's ruling opens the way for Bowden to seek possession of Mellinzeath from Mr and Mrs Smyth-Tyrrell.